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Preparation and characteristics of a novel nano-sized calcium carbonate (nano-CaCO.sub.3)-supported nucleating agent of poly(L-lactide).

INTRODUCTION

Poly(L-lactide) (PLLA) has attracted much attention because it is biomass-derived, biodegradable. biocompatible. and nontoxic to the environment and human body. Recent innovation on the production process has lowered significantly the production cost. which further stimulates the studies on its property and potential applications (1). However. PLLA exhibits a rather slow crystallization rate. which greatly limits its practical applications.

Addition of nucleating agent (NA) into PLLA matrix has been proved to he an effective approach to enhance the crystallization rate by lowering the surface free energy barrier toward nucleation (2- 5). Several nucleating agents have been developed and their effects on the crystallization behavior of PLLA have been widely studied (6-11). Talc was a widely used NA for PLLA (6), (7). It was shown that the crystallization half-time of PLLA could be reduced to less than 1 min when 1 wt% talc was added. Okamoto and coworkers used a low-molecular- weight aliphatic amide, namely, N, N-ethylenebis (12-hydroxy-steara-mide). as NA for PLLA and studied the crystallization kinetics and morphology of this PLLA/NA system (8). It was found that nucleation density and overall crystallization rate of PLLA could he greatly increased. Some hydrazide compounds reported by Kawamoto et al. can accelerate the crystallization of PLLA and the relationship between the chemical structure and nucleation efficiency of these kinds of NAs was studied (9). Li and Huneault investigated the effect of nucleation and plasticization on the crystallization of PLLA by adding talc, sodium stearate, and calcium lactate as potential nucleating agents (10), and the non-isothermal data showed that the combination of NA and plasticizer was necessary to develop significant crystallinity at high cooling rates. Besides aforementioned nucleating agents, stereocomplexation between PLLA and poly(D-lactic acid) (PDLA) was one of the most effective and promising methods for increasing crystallization rate of poly(lactic acid) (PLA)- based materials (12-28). It was found that the overall crystallization rate of PLA stereocomplex was much higher than that of pure PLLA or PDLA, clue to extremely high radius growth rate and density (number per unit area or volume) of stereocomplex spherulites and a very short induction period for the formation of stereocomplex spherulites compared to those of PLLA or PDLA spherulites. However, a high production cost of PDLA greatly limits the wide applications of the stereocomplexed PLLA materials.

Metal phosphonate is a kind of synthetic inorganic/organic hybrid material with a layered structure (29), (30). It was reported by Mitomo et al. that the metal phosphonate materials can accelerate the crystallization rate of PLLA (31). Very recently, layered metal phosphonate, zinc phenylphosphonate (PPZn), and reinforced PLLA composites were fabricated by a melt-mixing technique (32). PPZn showed excellent nucleating effects on PLLA crystallization. With incorporation of 0.02 wt% PPZn, PLLA can finish crystallization under cooling at 10 [degrees] C [min.sup.-1]. In our previous work (33), PLLA nucleated by layered metal phosphonates was prepared via melt blending using three kinds of layered metal phosphonates, i.e., PPZn, calcium phenylphosphonate (PPCa), and barium phenylphosphonate (PPBa). The morphology, crystallization, and biodegradation of PLLA nucleated by layered phenylphosphonates salts of different metal ions ([Zn.sup.2+], [Ca.sup.2+], and [Ba.sup.2+]) were investigated. It was found that PPZn, PPCa, and PPBa, served as effective nucleating agents, accelerate both nonisothermal and isothermal crystallization, and biodegradation of PLLA.

Although metal phosphonate possesses excellent nucleating effect on PLLA, which has attracted a great deal of interest, the lower toughness of nucleated PLLA compared with non-nucleated PLLA and the high cost of metal phosphonate would restrict its applications. If a good dispersion is achieved, rigid particles toughening will be more beneficial than rubber toughening since both stiffness and toughness can be increased by the former (34). Incorporating nano-[CaCO.sub.3] or organically modified layered silicates (OMLS) into a PLLA matrix has been studied extensively for improvements of toughness (35). Evidence of micromechanical deformation suggested that nano- [CaCO.sub.3] increased the strain-at-break of PLLA by massive crazing across the whole gauge length and the matrix eventually failed due to the coalescence of the microvoids. To improve the toughness of nucleated PLLA is an interesting subject of industry, science, and technology. In order to increase the nucleation efficiency and improve the mechanical performance of nucleated PLLA, supporting nucleating agent on nanoparticles would be a promising approach. Based on the preparation principles of high efficiency-supported catalyst of olefin polymerization and the nucleation mechanism of metal phosphonate, the nucleating agent supported on nano-[CaCO.sub.3] for PLLA was prepared by supporting PPCa on nano- [CaCO.sub.3] surface for the first time in this work. The supported nano-[CaCO.sub.3] may not only disperse nucleating agent active component effectively and increase the efficiency, but also increase the toughness and stiffness of nucleated PLLA due to the toughen and reinforce of nano- [CaCO.sub.3] (35). Therefore, it is significant for the nano-[CaCO.sub.3] supported nucleating agent to increase the nucleation efficiency, and spreading application of nucleated PLLA. The purpose of this article is to prepare and characterize the nano- [CaCO.sub.3] supported PPCa. The morphology, nonisothermal and isothermal crystallization kinetics, spherulite morphology, and crystalline structure of the nucleated PLLA by nano- [CaCO.sub.3]-supported PPCa were systematically investigated.

EXPERIMENTAL

Materials

PLLA (4032D) used in this study comprising around 98% L-lactide was a commercial product of Natureworks Co. Ltd., USA. It exhibited a density of 1.25 g [cm.sup.-3], a weight-average molecular weight ([M.sub.w]) of 207 kg [mol.sup.-1]. Phenylphosphonic acid (PPOA) with a melting point of about 163.3[degrees]C was purchased from Jiaxing Alpharm Fine Chemical Co. Ltd., China. The nano- [CaCO.sub.3] with an average primary particle size of about 70 nm was supplied by Solvay (Shanghai) Co., Ltd., China. It was coated with 3 wt% stearic acid. Its specific surface area is larger than 30 [m.sup.2] [g.sup.-1] (as per the manufacturer).

Synthesis of PPCa

PPCa was synthesized according to previous work (33).

Preparation of Nucleating Agent Supported on Nano-[CaCO.sub.3]

PPOA and nano-[CaCO.sub.3] were dried under vacuum at room temperature before use. The nucleating agent supported on nano-[CaCO.sub.3] was prepared by the impregnation method. The PPOA was mixed with nano-[CaCO.sub.3] in acetone solution with stirring at room temperature for 4 h. Then the mixture was subjected to centrifugation. After removal of the acetone, the white sediment was redispersed in acetone by stirring and centrifuging again, and the white sediment was collected. This dispersion-centrifugation-collection cycle was repeated three times to remove the unreacted PPOA. At last, the supported nano-[CaCO.sub.3] was dried in a vacuum oven at 80 [degrees] C to a constant weight. The prepared nucleating agent supported on nano-[CaCO.sub.3] was denoted as NAx, the x means the initial mass ratio of support/PPOA ([M.sub.Support]/[M.sub.PPOA]).

Sample Preparation

Before sample preparation, all the materials were adequately dried in a vacuum oven at appropriate temperatures. The PLLA nucleated by supported nucleating agents with the content of 1 wt%, 3 wt%, and 5 wt% with different [M.sub.Support]/[M.sub.PPOA] was melt compounded at 180 [degrees] C for 8 min using a Haake rheomix 600 internal mixer, the rotor speed was 50 revolutions per minute, and the total mixing weight per batch was about 60 g. For comparison, neat PLLA, PLLA nucleated by 1 wt% and 2 wt% PPCa, and PLLA filled by 1-5 wt% nano-[CaCO.sub.3] was also prepared in the same conditions, respectively. Then the samples were hot-pressed at 190 C for about 1 min followed by cold-press at room temperature to Form the film with thickness of about 1.0 mm for characterization.

Characterization

Fourier transform infrared (FTIR) spectra were recorded using a BIO-Rad Win-IR spectrometer in the range of 500-4000 [cm.sup.-1] with a resolution of 4 [cm.sup.-1].

The molecular weight parameters of neat PLLA and various samples were measured by gel permeation chromatography (GPC) using a Waters instrument (515 HPLC) equipped With a Wyatt interferometric refractometer. GPC columns were eluted with chloroform at 25 C at 1 mL. [min.sup.-1]. The molecular weights were calibrated with polystyrene standards. After melt processing, the molecular weights of neat PLLA, melt-processed PLLA, and PLLA containing 2.0 wt% PPCA. 5.0 wt% [CaCO.sub.3] and 5.0 wt% NA5 were investigated, which were 207. 195. 185. 188, and 192 kg [mol.sup.-1] with [M.sub.w]/ [M.sub.n] values of 1.73, 1.80. 1.70. 1.82, and 1.75, respectively. The molecular weight parameters characterization confirms that. under processing conditions, melt blending of PLLA with PPCA. [CaCO.sub.3] and nano-[CaCO.sub.3]-supported nucleating agent did not induce any dramatic drop of PLLA weight average molar mass by thermal degradation or hydrolysis of the polyester chains. A similar result in PLLA/PPZn blend was also reported by Pan et al. (32). A slight difference may be due to the different molecular weight of initial PLLA and melt processing conditions.

The morphology of the fractured surface which was prepared under liquid nitrogen was observed using a field emission scanning electron microscopy (SEM) (XL30 ESEM EEG. FEI Co.). The surface of the samples was coated with a thin layer of gold prior to the measurement.

Thermal analysis was performed using a TA instruments differential scanning calorimeter (DSC) Q20 with a Universal Analysis 2000. Indium was used for temperature and enthalpy calibration. All operations were performed under nitrogen purge. and the weight of the samples varied between 5 and 8 mg. In the case of nonisothermal melt crystallization, the samples were heated from the ambient temperature to 190 C at a heating rate of 50 C [min.sup.-1]. held for 2 min to erase the thermal history. and then cooled to 0 C at a cooling rate of 10 (7 [min.sup.-1]. Alter cooling from 190 C at a cooling rate of 10 C [min.sup.-1]. the samples were further heated to 190 C again from 0 C at a heating rate of 10 C [min.sup.-1] to investigate the subsequent melting behavior. The crystallization peak temperature was obtained from the cooling traces. The melting point temperature and melting enthalpy of samples were recorded irony the heating traces at a heating rate of 10 C [min.sup.-1] The enthalpy of crystallization and melt has been calculated from the enthalpy of crystallization and melt normalized to the PLLA content. In the case of isothermal melt crystallization experiment, the samples were heated from the ambient temperature to 190 C at a rate of 50 C [min.sup.-1]. held for 2 min. cooled to crystallization temperature ([T.sub.c] at a cooling rate of 45 C [min.sup.-1], and held until the isothermal crystallization was over. The crystallization temperature chosen in this work were 130 C and 140 C, respectively. The exothermal traces were recorded for data analysis.

An optical microscope (P0M) (Leica DM2500 P) equipped with a temperature controller (Linkam LTS 350) was used to investigate the spherulitic morphology. The samples were first annealed at 190 C for 2 ruin to erase any thermal history and then cooled to 140 C at a cooling rate of 50 C [min.sup.-1] for 50 min.

Wide-angle X-ray diffraction (WAXD) patterns were recorded using a Rigaku model Dmax 2500 X-ray diffractometer. WAXD patterns were recorded from 5 to 40 at 3 [min.sup.-1]

RESULTS AND DISCUSSION

Crystallization Behavior and Melting Characteristies of PPOA and the Supported Nucleating Agent

In order to investigate the chemical reaction between PPOA and nano-[CaCO.sub.3] crystallization behaviors and melting characteristics of PPOA and the supported nucleating agent were characterized by DSC. The DSC curves of PPOA and the supported nucleating agent are shown in Fig. 1. It can he observed that the PPOA has a melting peak at 167.3 C with a melt enthalpy being around 170.8 J [g.sup.1] and and a crystallization peak at 132.6 C with a crystallization enthalpy being around 156.6 J [g.sup.-1] For the supported nucleating agent NA5 prepared by impregnation method, no discernible melting and crystallization peaks of PPOA were observed during heating and cooling process.

Figure 2 shows the FTIR spectra of nano- [CaCO.sub.3] PPOA, and the supported nucleating agent NA5. In the spectra of PPOA, the P--C stretching band characteristic of the P--[C.sub.6] [H.sub.15] group and the P=0 stretching band is present at 1439 [cm.sup.-1] and 1222 [cm.sup.-1], respectively (36). In addition, the C--H out-of- plane deformation bands of the monosubstituted benzene ring are located at 756 and 696 [cm.sup.- 1], respectively (36). For the supported nucleating agent NA5, the C--H out-of-plane deformation bands of the monosubstituted benzene ring (756 and 696 [cm.sup.-1] of PPOA can be clearly observed, indicating that the chemical reaction took place between nano-[CaCO.sub.3] and PPOA to form PPCa on the surface of nano-[CaCO.sub.3] during impregnation process, the possible chemical reaction between PPOA and nano-[CaCO.sub.3] is shown in Scheme 1.

Figure 3 shows WAXD patterns of nano- [CaCO.sub.3] the supported nucleating agent NA5, and PPCA. The WAXD patterns displayed a 2[theta] value of 5.6[degrees] and an interlayer d spacing of 1.528 nm for PPCa [estimated from the (010) reflection]. A very similar interlayer d spacing of PPCa supported on nano-[CaCO.sub.3] NA5 was observed, indicating the crystal structure of PPCa was not altered after supporting.

Morphology and Dispersion of the Supported Nucleating Agent in the PLLA Matrix

The dispersion of NAs in the polymer matrix as well as interfacial interactions between the polymer matrix and NAs plays an important role in affecting the crystallization behavior of polymers. A homogeneous dispersion of NAs and strong interfacial interactions between polymer matrix and NAs can effectively enhance the crystallization of polymers. To reveal the structure and dispersion of the supported nucleating agent in the PLLA matrix., the fracture surfaces of the samples were investigated by SEM. Figure 4 shows the fracture surfaces of neat PLLA, PLLA containing 1.0 wt% PPCA, and PLLA containing 1.0 wt%, 3.0 wt%, and 5.0 wt% NA5. The layered structure of PPCa with severe aggregation can be clearly seen in Fig. 4b. In addition, it was found that some of PPCa were broken apart, and debonded from the PLLA matrix. Such typical breakage phenomenon of the PLLA/PPCa blending indicated that the interfacial adhesion between PPCa and PLLA matrix was poor. For PLLA containing the supported nucleating agent in Fig. 4c and d, only white specks existed on the PLLA matrix surface, and this feature was more obvious with increasing the content of the supported nucleating agent. Comparing with the surface of neat PLLA in Fig. 4a, it can be seen that the white specks should be the supported nucleating agent. Generally, nano-[CaCO.sub.3] easily aggregates due to the particle--particle interaction, and the aggregated nano-[CaCO.sub.3] particles can also be found in PLLA/nano-[CaCO.sub.3] blending as shown in Fig. 4. However, the nano-[CaCO.sub.3] particles were dispersed evenly in the PLLA matrix even with 5 wt% loading as a result of sufficient shear force imposed during the melt compounding process and stearic acid coated surface. Moreover, it was obvious that the dispersion and interfacial adhesion of PLLA incorporated with the supported nucleating agent were better than those of PLLA incorporated with PPCa (as seen from Fig. 4). Therefore, some performances of PLLA incorporated with the supported nucleating agent may be superior to those of PLLA incorporated with PPCa, which will be discussed in the following sections.

Effect of Nano-[CaCO.sub.3] and PPCa on Crystallization Behavior and Melting Characteristics of PLLA

The supported nucleating agent containing nano- [CaCO.sub.3] and nucleated active component PPCa was prepared. In the preparation process, the chemical reaction took place between PPOA and nano -[CaCO.sub.3] to form the PPCa nucleating agent on nano-[CaCO.sub.3] surface. Thus, the influence of nano-[CaCO.sub.3] and PPCa on crystallization behavior and melting characteristics of PLLA was investigated first. Figure 5 shows the DSC crystallization and melting curves of P1AA nucleated by PPCA and filled by nano-[CaCO.sub.3], and the corresponding data are listed in Table 1. The degree of crystallinity has been calculated according to the equation [W.sub.c] = 100 x ([DELTA][H.sub.m] [DELTA][H.sub.cc])/[DELTA] [H.sup.0.sub.m] where [W.sub.c]. is the degree of crystallinity, [DELTA][H.sub.cc] is the specific enthalpy or melting., is the specific enthalpy of cold crystallization. [DELTA][H.sub.0.sup.m] is the heat of fusion or 100% crystalline PLLA, which is 93 J [g.sup.-1] as reported in the literature (37).

It can be clearly seen from Fig. 5 that there is no discernible crystallization peak at a cooling rate of 10 C [min.sub.-1] for neat PLLA and PLLA filled by nano-[CaCO.sub.3]: however, a melt crystallization peak at 114.9 C of PLLA nucleated by PPCA can be observed. The subsequent melting characteristics of neat PLLA, PLLA nucleated by PPCA and filled by nano-[CaCO.sub.3] after cooling from the melt at 10 C [min.sup.-1] are much more different from each other. It can be seen from Fig. 5h that neat PLLA exhibited a glass transition temperature ([T.sub.g]) of around 60 C and a cold crystallization peak temperature ([T.sub.cc]) of around 111.7 C. Incorporation of nano-[CaCO.sub.3] did not show great effect on the [T.sub.g] and [T.sub.cc] of PLLA. However, PLLA nucleated by PPCA did not show cold crystallization during heating to the melt, indicating the crystallization can be finished at a cooling rate of 10 C [min.sup.1]. Therefore, the PPCA is a nucleating agent of PLLA with high nucleating efficiency.

Effect of the Supported Nucleating Agent on Crystallization Behavior and Melting Characteristics of PLLA

Figure 6a shows the DSC crystallization curves of PLLA nucleated by supported nucleating agents. Analysis of the exotherms based on the characteristic parameters (38-40) of the exotherms is presented below to distinguish the effect of the supported nucleating agents on overall crystallization process (combined effect of nucleation and growth steps), crystallinity, nucleation rate, and crystal size distribution. Exotherm parameters, as illustrated in previous work (38-40), are the temperature of onset of crystallization ([T.sub.onset]), at the point where the exotherm initially departs from the baseline, the peak temperature ([T.sub.p]), which may be referred to as the characteristic temperature of the overall crystallization process, the initial slope ([S.sub.i]), which is measured as the slope of the initial linear portion of the exotherm peak, the width of the exotherm ([DELTA]W) and the enthalpy of crystallization normalized to the PLLA content ([DELTA][H.sub.c]), which is related to the degree of crystallinity.

TABLE 1. DSC results for various samples.

Sample         [T.sup.a.sub.cc]  [DELTA] [H.sup.a.sub.cc]   [T.sub.m1]
                   ([degrees]C)            (J  [g.sup.-1]  ([degrees]C)

Neat PLLA                 111.7                      39.2         160.9

1.0%                         --                        --         164.1
PPCA/PLLA

2.0%                         --                        --         164.3
PPCA/PLLA

1.0%                      111.7                      36.7         162.3
[CaCO.sub.3]/
PLLA

3.0%                      113.9                      35.4         163.1
[CaCO.sub.3]/
PLLA

5.0%                     1 15.0                      37.4         163.4
[CaCO.sub.3]/
PLLA

1.0% NA5/PLLA                --                                   165.0

3.0% NA5/PLLA                --                        --         165.4

5.0% NA5/PLLA                --                        --         165.2

1.0%                         --                        --         164.2
NA10/PLLA

3.0%                         --                        --         165.4
NA10/PLLA

5,0%                         --                        --         166.3
NA10/PLLA

1.0%                         --                        --         165.6
NA20/PLLA

3.0%                         --                        --         166.2
NA20/PLLA

5.0%                         --                        --         165.3
NA20/PLLA

1.0%                         --                        --         165.7
NA40/PLLA

3.0%                         --                        --         164.6
NA40/PLLA

5.0%                         --                        --         166.0
NA40/PLLA

Sample          [T.sub.m2]         [DELTA]   [W.sub.c]
              ([degrees]C)       [H.sub.m]         (%)
                             (J [g.sup.-1]

Neat PLLA             167.9           47.4        8.8

1.0%                  168.8           45.4       48.8
PPCA/PLLA

2.0%                  168.9           45.3       48.7
PPCA/PLLA

1.0%                  168.8           44.3        8.2
[CaCO.sub.3]/
PLLA

3.0%                  169.0           43.5        8.7
[CaCO.sub.3]/
PLLA

5.0%                  168.9           44.9        8.1
[CaCO.sub.3]/
PLLA

1.0% NA5/PLLA            --           45.4       48.8

3.0% NA5/PLLA            --           47.1       50.6

5.0% NA5/PLLA            --           47.2       50.8

1.0%                     --           45.3       48.7
NA10/PLLA

3.0%                     --           47.4       51.0
NA10/PLLA

5,0%                     --           44.7       48.1
NA10/PLLA

1.0%                     --           42.9       46.1
NA20/PLLA

3.0%                     --           46.1       49.6
NA20/PLLA

5.0%                     --           42.9       46.1
NA20/PLLA

1.0%                     --           45.6       49.0
NA40/PLLA

3.0%                     --           43.7       47.0
NA40/PLLA

5.0%                     --           45.0       48.4
NA40/PLLA

(a.) Cold crystallization at a heating rate of
lO [degrees] C [min.sup.-1].


Values of crystallization parameters determined from the DSC thermograms are shown in Table 2. It is apparent that the supported nucleating agent significantly increases the [T.sub.p] of PLLA. PLLA with the addition of 1 wt% supported nucleating agent NA40 has a [T.sub.p] of 112.7 C and the [T.sub.p] of PLLA increases with increasing the content of the supported nucleating agent. Moreover, compared with PLLA nucleated by PPCA, an increase of [S.sub.i] and a decrease of At,. on addition of the supported nucleating agents were observed. An increase of dispersion of nucleating agent will increase [S.sub.i], and cause almost simultaneous occurrence of nucleation. resulting into most crystallites grown simultaneously, hence producing small size and narrow size distribution of spherulites. The degree of crystallinity did not show identical trend with the trend of variation of [S.sub.i]. This may he due to the fact that nucleation is affected by nucleating agent while the growth is affected by molecular chain mobility.

TABLE 2. DSC results for various samples.

Sample      [T.sub.p]  [T.sub.onset]          [S.sub.i]  [DELTA]W
                  (C)            (C)  (arbitrary  unit)       (C)

Veal PLLA          --             --                 --        --

1.0% PPCA/      111.2          125.2               0.23       8.0
PLLA

2.0% PPCA/      114.9          124.3               0.23       5.5
PLLA

1.0% NA5/       115.7          125.3               0.25       4.7
PLLA

3.0% NA/        118.1          126.3               0.33       4.5
PLLA

5.0% NA5        119.3          126.5               0.46       4.0
/PLLA

1.0% NA10/      115.5          122.3               0.25       4.8
PLLA

3.0% NA10/      117.1          123.5               0.28       4.6
PLLA

5.0% NA10/      117.4          124.3               0.38       4.5
PLLA

1.0% NA20/      113.1          120.8               0.23       4.8
PLLA

3.0%            116.2          122.8               0.26       4.7
NA20/PLLA

5.0% NA20/      116.8          123.8               0.37       4.6
PLLA

1.0% NA40/      112.7          119.8               0.23       4.9
PLLA

3.0% NA40/      115.0          121.8               0.24       4.8
PLLA

5.0% NA40/      116.0          122.9               0.36       4.6
PLLA

Sample      [DELTA][H.sub.c]
              (J [g.sup.1])

Veal PLLA                 --

1.0% PPCA/              37.7
PLLA

2.0% PPCA/              42.1
PLLA

1.0% NA5/               42.6
PLLA

3.0% NA/                43.7
PLLA

5.0% NA5                44.3
/PLLA

1.0% NA10/              41.7
PLLA

3.0% NA10/              44.6
PLLA

5.0% NA10/              40.1
PLLA

1.0% NA20/              40.0
PLLA

3.0%                    43.2
NA20/PLLA

5.0% NA20/              40.7
PLLA

1.0% NA40/              42.6
PLLA

3.0% NA40/              43.7
PLLA

5.0% NA40/              41.5
PLLA


Figure 6b shows the DSC melting curves of PLLA nucleated by the supported nucleating agents. The related data can he seen in Table 1. The melting curves exhibited no cold crystallization peaks, which were similar to that of PLLA nucleated by PPCA. For samples of PLLA and PLLA with incorporation of PPCa and [CaC0.sub.3], two melting peaks ([T.sub.m1] and [T.sub.m2]) were observed. The double melt phenomenon was attributed to a melt recrystallization-remelting process upon heating. The lower temperature peak was ascribed to the melting of primary crystals and the higher temperature peak or shoulder corresponded to the melting of the recrystallized crystals (41-45). When the supported nucleating agent was used, the crystallization peak temperature ([T.sub.p]) was relatively higher and the more perfect crystals were formed (32), thus the samples melt without the melt-recrvstallization process, and a single endothermic peak can he observed.

In order to quantitatively evaluate the enhanced nucleating activity of supported nucleating agents, we assume that all acid is reacted; then the concentration of PPCa in the nano-[CaC0.sub.3] supporting material could be calculated. Figure 7 shows the curves of the content of PPCa in PLLA matrix versus [T.sub.p] for PLLA nucleated by PPCa and the supported nucleating agent. Although the crystallization behavior and melting characteristics of PLLA nucleated by the supported nucleating agent were similar to that of PLLA nucleated by PPCA, the content of nucleating agent active component PPCA in PLLA nucleated by supported nucleating agent was less. The dispersion of the supported nucleating agent in PLLA matrix was better than that of PPCa in PLLA matrix (as seen from Fig. 4). Thus, the nano-[CaCO.sub.3] support can direct specific crystal surfaces (more effective for nucleation) of nucleating agent to be exposed to the crystallizing PLLA, which greatly improved the crystallization behavior of PLLA.

Therefore. the supported nucleating agent possesses high nucleating ability.

As introduced in the Experimental Section. the overall isothermal crystallization of various samples Was investigated by DSC in the temperature of 130 C and 140 C. respectively. In the isothermal crystallization process. the relative degree of crystallinity ([X.sub.t]) at crystallization time is defined as the ratio of the area under the exothermic curve between the onset crystallization time and the crystallization time t to the whole area under the exothermic curve from the onset crystallization time to the end crystallization time. Figure 8 shows [X.sub.t] versus t for neat PLLA and PLLA containing 5 wt% nano-[CaCO.sub.3]. 2 wt% PPCa. and 5 wt% NA5 crystallised at 130 C and 140 C, respectively. It is clear that the addition of the supported nucleating agent enhanced the isothermal melt crystallization of PLLA compared with other samples.

The Avrami equation is frequently employed to analyze the isothermal crystallization kinetics of polymers. according to which [X.sub.t] dependent crystallization time t can be expressed as (46), (47):

1 - [X.sub.t] = exp(-[kt.sub.n]) (1)

where n is the Avrami exponent dependent on the nature of nucleation and growth geometry of the crystals, and k is the overall rate constant associated with both nucleation and growth contributions. The linear form of Eq. I can be expressed as follows:

log[-ln(1 - [X.sub.t])] = logk + nlogt (2)

The Avrami parameters n and k can be obtained from the slopes and the intercepts, respectively. Figure 9 shows Avrami plots of log[-ln(1 - [X.sub.t])] against log t for neat PLLA and PLLA containing 5 wt% nano-[CaCO.sub.3], 2 wt% PPCa, and 5 wt% NA5 crystallized at 130 [degrees] C and 140 [degrees] C, respectively. The obtained Avrami parameters of various formulations are summarized and listed in Table 3. From Table 3, it can be seen that the values of Avrami exponent n ranges from 2.3 to 3.0, whereas it is almost insensitive to the addition of nano-[CaCO.sub.3], PPCa, and the supported nucleating agent, indicating that the crystallization mechanism may not change. The DSC results reported herein are consistent with the spherulitic morphology and growth studies in the following section. On the other hand, the values of k decrease with increasing crystallization temperature for all the samples, which indicated that the crystallization process studied in this research was a nucleation-controlled process due to the low supercooling ([T.sup.0.sub.m] - [T.sub.p] (48), where [T.sup.0.sub.m] denoted the equilibrium melting point. For a given supported nucleating agent. the values of k increase with increasing contents. The increase in the values of k suggests that the incorporation of supported nucleating agent accelerated the overall crystallization process of PLLA as compared with neat PLLA, which should be attributed to the heterogeneous nucleation effect of the supported nucleating agent on the crystallization of PLLA.

TABLE 3. Crystallization kinetic parameters for various
samples.

                      [T.sub.c] = 130 C

Sample          n               k   [t.sub.0.5]     n
                     [min.sup.-n])        (min)

Neat PLLA      2.4         0.00023        28.74   2.4

1.0%           2.8            0.25         1.45   2.5
PPCA/PLLA

2.0%           2.4            0.66         1.02   2.5
PPCA/PLLA

1.0%           2.6          0.0024         8.80   2.6
[CaC0.sub.3]/
PLLA

3.0%           2.5          0.0028         9.10   2.5
[CaCO.sub.3]/
PLLA

5.0%           2.3          0.0030        10.70   2.4
[CaC0.sub.3]/
PLLA

1.0% NA5/PLLA  2.4            0.44         1.20   2.5

3.0% NA5/PLLA  2.5            0.78         1.00   2.7

5.0% NA5/PLLA  2.6            0.87         0.92   2.3

1.0%           2.6            0.15         1.80   3.0
NA10/PLLA

3.0%           2.3            0.76         0.96   2.7
NA10/PLLA

5.0%           2.3            0.78         0.95   2.5
NA10/PLLA

1.0%           2.6            0.15         1.80   2.7
NA20/PLLA

3.0%           2.5            0.25         1.50   2.8
NA20/PLLA

5.0%           2.3            0.40         1.27   2.6
NA20/PLLA

1.0%           2.4            0.20         1.68   2.7
NA40/PLLA

3.0%           2.3            0.22         1.64   2.5
NA40./PLLA

5.0%           2.6            0.20         1.61   2.3
NA40/PLLA

                      [T.sub.c] = 140 C

Sample                     k   [t.sub.0.5]
               ([min.sup.-n])        (min)

Neat PLLA            0.000028        70.56

1.0%                    0.013         4.91
PPCA/PLLA

2.0%                    0.014         4.58
PPCA/PLLA

1.0%                 0.000031        47.00
[CaC0.sub.3]/
PLLA

3.0%                 0.000037        51.17
[CaCO.sub.3]
PLLA

5.0%                 0.000069        46.50
[CaC0.sub.3]/
PLLA

1.0% NA5/PLLA          0.0030         8.80

3.0% NA5/PLLA          0.0050         6.20

5.0% NA5/PLLA           0.020         4.38

1.0%                  0.00035         9.70
NA10/PLLA

3.0%                   0.0029         7.59
NA10/PLLA

5.0%                   0.0037         8.10
NA10/PLLA

1.0%                  0.00020        20.40
NA20/PLLA

3.0%                  0.00060        11.80
NA20/PLLA

5.0%                   0.0016        10.35
NA20/PLLA

1.0%                  0.00013        23.90
NA40/PLLA

3.0%                  0.00063        16.46
NA40./PLLA

5.0%                   0.0025        11.22
NA40/PLLA


The half-time crystallization time ([t.sub.0.5]), which is defined as the half-period (i.e., 50%) crystallization) from the onset of crystallization to the end of crystallization, is an important parameter for the discussion of isothermal crystallization kinetics. The value of [t.sub.0.5] can he calculated according to the following equation:

[t.sub.0.5] = [(ln 2/k).sup.l n] (3)

The values of [t.sub.0.5] are calculated by Eq. 3 and listed in Table 3. too. Figure 10 shows the curves of the content of PPCa in PLLA matrix versus [t.sub.0.5] for PLLA nucleated by PPCa and supported nucleating agents. It is clear from Table 3 that the overall crystallization rate was faster for PLLA incorporated of the supported nucleating agent than that of neat PLLA. With a similar content of nucleating agent active component PPCA, the overall crystallization rate of PLLA incorporated of the supported nucleating agent was faster than that of PLLA incorporated of PPCa. The difference in the overall crystallization rate may be related to the following factors. Although both the supported nucleating agent and PPCa act as effective NAs to enhance the crystallization rate of PLLA. the nucleating ability and the influence on the overall crystallization process are different. The crystallization behavior of polymers in the presence of NAs is affected by many factors. such as composition. interfacial interactions, size. and distribution. It is expected that an evenly distributed. good interfacial interactions may he contributed to improved nucleating efficiency. The dispersion and interfacial interactions of the supported nucleating agent were much better than those of PPCa throughout the PLLA matrix as evidenced by SEM observations in the previous section. Therefore, the supported nucleating agent is more effective to enhance the crystallization of PLLA compared to PPCa.

The effect of the presence of nano-[CaCO.sub.3]. PPCA, and supported nucleating agent on the spherulitic morphology of PLLA was studied by POM. Figure 11 presents the spherulitie morphology crystallized at 140 C for 50 min. As for neat PLLA, well-developed spherulites with a size of roughly 200 [micro]m in diameter can be observed. it is clear that the size of spherulites becomes smaller in the presence of the supported nucleating agent, indicative of the increase of nucleation density. Moreover, the diameter of spherulites decreases with increasing the content of nucleating agent active component PPCA. whereas the nucleation density increases in the reverse trend. Spherulitic morphology studies indicated that the nucleation density of PLLA was greatly improved as a result of the presence of the supported nucleating agent in PLLA matrix, which were well consistent with the DSC results in the previous section. These results revealed clearly that a good dispersion and interfacial interactions of supported nucleating agent in the PLLA matrix influenced efficiently the spherulitic morphology and the overall crystallization process of PLLA.

Figure 12 illustrated the WAXD patterns of various formulations, which were crystallized at 120 C for 2 h to ensure high level of crystallinity. PLLA can crystallize in [alpha] [beta]. and [gamma] forms. The [alpha] form of PLLA with a limiting disordered modification was recently found and defined as the [alpha] form (49). The most common polymorph of PLLA is the [alpha] form, which is believed to grow under the normal conditions such as the melt, cold, or solution crystallization. The [beta] form is usually formed upon stretching their [alpha] counterparts at high temperature and high-draw ratio. The [gamma] form can be obtained via epitaxial crystallization on the hexamethylbenzene (HMB) substrate (49). In the present case, the samples for the WAXD experiment were prepared through crystallization from the melt; therefore the samples should crystallize in [alpha] form. As shown in Fig. 12, neat PLLA exhibits three main characteristic diffraction peaks at around 15.0 [degrees], 16.8[degrees], and 19.2[degrees], corresponding to (010), (200), and (203), which are the typical diffraction peaks of [alpha] form as reported previously (32), (48). The WAXD patterns of the PLLA incorporated of nano-[CaCO.sub.3], PPCA, and the supported nucleating agent were almost the same to those of the corresponding neat PLLA samples, indicating that incorporation of nano-[CaCO.sub.3], PPCA, and nano-[CaCO.sub.3] supported nucleating agent did not alter the crystal structures after crystallization at 120 [degrees]C.

CONCLUSIONS

In view of both the preparation principle of high efficiency-supported catalyst of olefin polymerization and the nucleation mechanism of PPCa, we prepared supported nucleating agent for PLLA for the first time. A high efficiency nucleating agent was prepared by supporting PPCa On nano-[CaCO.sub.3] surface. The supported nucleating agent exhibited higher nucleation ability compared with PPCa. Addition of the supported nucleating agent enhanced the crystallization rate of PLLA. The new nucleating agent can he used in lesser amounts to obtain the same extent of nucleation.

Correspondence to: Changyu Han: e-mail: cyhan@ciae.jl.cn or Lisong Dong; e-mail: dongls@eiae.jl.cn

Contract gram sponsor: National Natural Science Foundation of China: contract grant number: 50703042: contract grant sponsor: Jilin Province Science and Technology Agency; contract grant number: 20116025.

DOI 10.1002/pen.23095

Published online in Wiley Online Library (wileyonlinelibrary.com). [C] 2012 Society of Plastics Engineers

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Lijing Han, (1), (2) Changyu Han, (1) Junjia Bian, (1) Yijie Bian, (1), (2) Haijuan Lin, (1), (2) Xuemei Wang, (1), (2) Huiliang Zhang, (1) Lisong Dong (1)

(1.) Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

(2.) Graduate School, Chinese Academy of Sciences, Beijing 10080, China
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Author:Han, Lijing; Han, Changyu; Bian, Junjia; Bian, Yijie; Lin, Haijuan; Wang, Xuemei; Zhang, Huiliang; D
Publication:Polymer Engineering and Science
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Geographic Code:9CHIN
Date:Jul 1, 2012
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