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Temperature influence on the behavior of polysulfone-b-poly(alkylene oxide)-b-poly(dimethylsiloxane) triblock copolymers in a selective solvent.

INTRODUCTION

Copolymerization is a versatile method to tailor materials for specific uses because of a synergistic combination of various properties of different types of macromolecular chains. Thus, many investigations were carried out continuously for designing copolymers with controlled architecture in order to obtain materials with targeted properties by changing the composition, the length of blocks, or segment sequences. Much interest has been directed towards studying the synthesis, solution behavior, and physical properties of block copolymer materials [1] because their architecture can be controlled to produce novel and more cost effective materials. Thus, the block copolymers are technologically and economically important as well as an interesting area of study in their own right. Copolymers present a high interest from both fundamental and practical point of view.

In dilute solutions, the solvent quality determines the hydrodynamic dimensions of macromolecular chains and thus it has a high influence on different parameters such as: intrinsic viscosity [2-4], preferential and total adsorption coefficients [2], optical properties [2-5], Flory-Huggins interaction parameter [6-8].

The incorporation of polysiloxanes as "soft" segments into high-performance polymeric materials, such as polysulfones, has been of high interest because of the combination of very different properties, interesting for a series of special applications [9-11]. Owing to their special properties, poly(dimethylsiloxane)s (PDMS) represent a group of quite unique polymers. They present very important physical and chemical properties: high chain flexibility, very low glass transition temperature, very good resistance at temperature, oxidant agents and ultraviolet radiation, good gas permeability, and biocompatibility, but they exhibit rather poor mechanical properties [3]. Polysulfones (PSU) are considered as high-performance polymers because of their excellent thermal properties, good resistance to inorganic acids and bases, and outstanding hydrolytic stability against hot water and steam sterilization [12, 13]. Depending on the amount of siloxane units in the PSU-b-PDMS copolymer and the average molecular weights of siloxane segments, the properties take into account low glass transition temperature, good elastomeric properties, enhanced toughness, and high gas permeabilities [9]. Thus, polysulfone block copolymers containing poly(dimethylsiloxane) put together very different properties [9-11] and consequently find various specialty applications: membranes (gas and liquid separation), medical purposes or biomaterials, food service, etc. Furthermore, the introduction of hydrophobic and hydrophilic chains, such as poly(alkylene oxide)s (PAO) containing blocks of propylene oxide and of ethylene oxide into PSU-b-PDMS backbone can generate new types of materials with specific properties. PAO are generally nonionic surfactants with excellent wetting, antifoaming properties, nonbiodegradability and weak mechanical strength. Because of the hydrophilic/hydrophobic balance, such block copolymers can undergo self-assembly into micelles at a low critical concentration. This balance can be adjusted by introducing some chains with hydrophobic or hydrophilic segments [14].

The present paper reports the synthesis of a series of block copolymers containing polysulfone, poly(alkylene oxide) and poly(dimethylsiloxane). Samples with different compositions and molecular weights were investigated in dilute solutions of 1,2-dichloroethane (DCE) at temperatures between 20[degrees]C and 75[degrees]C by viscometry and UV absorption spectroscopy.

EXPERIMENTAL

For the present study, the alyl end-capped polysulfone-b-poly(alkylene oxide) denoted PSU-b-PAO (sample 6) was prepared by the condensation reaction of chlorine-terminated PSU (sample I 9) and alyl-PAO-OH. Triblock copolymer samples (PSU-6-PAOb-PDMS) were prepared by addition of preformed [alpha],[omega]-bis(hydrogensilyl) poly(dimethylsiloxane) oligomers (denoted PDMS) to PSU-b-PAO using Pt as catalyst (Scheme 1), following the proce- I dure described elsewhere [15, 16]. The solvents were removed by heating up to 180[degrees]C with a constant purge of nitrogen. The unreacted PSU was removed by washing the precipitate with 20/80 (vol/vol) methanol/dimethylformamide. The unincorporated PDMS was removed by extraction with petroleum ether. Finally, the samples were dried in a vacuum oven at 50[degrees]C for 24 h.

Five copolymer samples (samples 1-5, Table 1) were selected and studied in comparison with the PSU-b-PAO copolymer used as precursor (sample 6), PDMS (sample 7), PAO (sample 8, E[O.sub.21-] h-P[O.sub.21], EO = ethylene oxide, PO = propylene oxide) and PSU (sample 9). The molar composition of copolymer samples determined from [sup.1]H NMR spectra is given in Table 1 which also contains the molecular weights determined either by GPC, or end group titration.

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Viscometric measurements were carried out in dichlorethane in the temperature range of 20[degrees]C-75[degrees]C using an Ubbelohde suspended level viscometer (type 0a, with a capillary diameter of 0.53 mm and a constant K of 0.005). The flow times for the solvent at different temperatures were in the range 90-142 seconds. Huggins and Kraemer equations were used to analyze the viscometric data and the values of the intrinsic viscosity, [[eta]], were determined.

UV absorption spectra were recorded in a Specord M42 Carl Zeiss Jena spectrophotometer equipped with thermoelectric cell holder and a temperature controller. 10 mm path length quartz cells fitted with PTEE stoppers were utilized for all measurements. The samples were dissolved in 1,2-dichloroethane and investigated after 24 hours.

Transmission electron microscopy (TEM) study was performed on a TESLA BS 513A electron microscope at an acceleration voltage of 80 kV. Thin films used for electron microscopy measurements were prepared by casting their 1% (by weight) dichloroethane solutions. The polymer films were placed on electrolytic grids coated with collodium-carbon and dried under vacuum at room temperature for several days.

RESULTS AND DISCUSSION

PSU chains give an intense absorption band located at 266 nm, while PDMS and PAO did not absorb in the 220-320 nm domain. In this study, the absorption spectra of PSU (sample 9), PSU-b-PAO-b-PDMS (samples 1-5), and PSU-WAO (sample 6) in 1,2-dichloroethane were recorded at different temperatures and the relative absorbance [A.sub.max]/[A.sup.o] was calculated for each sample (where [A.sub.o] and Amax are the values of maximum absorbance at 20[degrees]C and at a given temperature, t, respectively, when the temperature varies from 25[degrees]C to 75[degrees]C). Figure 1 shows the dependence of the relative absorbance for PSU sample and PSU-b-PAO-b-PDMS block copolymers as a function of temperature. For all samples, the ratio [A.sub.max]/[A.sub.o] decreases with increasing the temperature. Previously, for high molecular weight poly(methyl methacrylate) solutions, it was observed that the UV absorbance-temperature dependence lie on two straight lines with different slopes, intersecting at the temperature at which conformational transitions occur, as evidenced also by viscometry [17-19]. In Fig. 1, a change in slope can be depicted around 55[degrees]C for PSU (sample 9) and PSU-b-PAO diblock copolymer (sample 6), but bellow and above this temperature the dependences are more or less linear. In the presence of PDMS blocks, the temperature dependences of [A.sub.max]/[A.sub.o] are not linear, even for narrow domains of temperature.

Examination of UV absorption spectra of copolymer solutions shows that the maximum absorbance is shifted slightly to the shorter wavelengths for all triblock copolymer samples, including PSU-b-PAO diblock copolymer, as compared with PSU, which can be explained in terms of some conformational constraints of the macromolecular chains. The same observation was reported for PSU-b-PDMS diblock copolymers [11]. The absorption bands are attributed the phenyl rings of the polysulfone moieties and the changes observed in the absorption spectra are due to the interactions exhibited with PAO or PDMS blocks.

UV spectroscopy is an adequate technique to study the conformational transitions of copolymers as a function of temperature, being used previously for graft [20, 21] and block [11, 22] copolymers. The variation of the electronic transition energy <2767024H_TB001> (E = c x h/[lambda], c is the speed of light, h is the Planck's constant, and [lambda] is the wavelength for which [A.sub.max] was registered) with the temperature can offer reliable information in discussing the conformational changes because of the sensitivity of the method. In our case, some displacements of the maximum absorbance to lower wavelengths were observed for the triblock copolymer samples as the temperature increases.

Figure 2 shows the the variation of absorbed energy (E) in the UV measurements with temperature calculated from the UV spectra recorded for the block copolymers and PSU samples. For PSU, up to 55[degrees]C, E is nearly constant, showing small variations at 20[degrees]C, 25[degrees]C, and 55[degrees]C. Above 55[degrees]C, E presents a continuously increase. By introducing PAO into PSU backbone (sample 6), the values of E increases and its temperature dependence is different: a continuous increase bellow 55[degrees]C where a maximum can be observed. Higher PDMS block lengths determine an increase of E and temperature dependences are similar to those observed for the sample 6, with maxima at 55[degrees]C for all triblock copolymer samples.

In a previous paper [11], it was observed that the shape of the electronic absorption spectra of PSU in the PSU-b-PDMS diblock copolymers does not present any important differences as compared with the UV spectra of PSU homopolymer. The presence of very flexible PDMS chains, which are not soluble in DCE, determines an increase of E between 20[degrees]C and 55[degrees]C and the copolymers adopt a segregated conformation, well-described by core and shell modell [1, 11, 23]. The flexible segments of PDMS are located inside the core. PSU blocks present an open conformation in solution because of the favorable PSU-DCE interactions and they are located into the shell. In the case of PSU-b-PAO-b-PDMS samples, the connections between PDMS and PSU blocks are assured by the flexible short chains of PAO.

According to Fig. 2, above 55[degrees]C, E decreases for all triblock copolymer samples and this is probably because of the higher contribution of thermal motion which influences the dynamics of flexible segments. By comparing the data obtained for different samples, it was observed that the values of the electronic transition energy are influenced in a higher extent by PAO content than PDMS one. By increasing the temperature, the copolymer chains gain enough energy to overcome the potential barrier between low-energy and high-energy rotational isomeric states and undergo a transition between these two states [11].

The behavior of the triblock copolymers in DCE solution is much more complicated. This is because of the fact that the different blocks present different solvent affinity. For a better understanding of the temperature influence on the triblock copolymer conformation, viscometric investigations were carried out between 20[degrees]C and 75[degrees]C in dilute DCE solution. The intrinsic viscosity, [[eta]], was determined by double extrapolation of the experimental data to infinite dilution according to the Huggins and Kraemer methods:

Huggins:

[[eta].sub.sp]/c = [eta] + [k.sub.H] [[[eta]].sup.2]c (1)

Kraemer:

ln[[eta].sub.rel]/c = [eta] + [k.sub.K] [[[eta]].sup.2]c (2)

where [[eta].sub.sp] is the specific viscosity, [[eta].sub.rel]--relative viscosity, c--polymer concentration, [k.sub.H], [k.sub.K]--Huggins and Kraemer constants, respectively.

Figure 3 shows as an example the Huggins and Kraemer plots for sample 1 at 30[degrees]C.

The dependences of the intrinsic viscosity as a function of temperature are shown in Fig. 4. A first observation is that the viscosity values are influenced by the copolymer composition in a higher extent as compared with the molecular weight effect. For PAO copolymer (sample 8) and PSU (sample 9), a continuous decrease of the intrinsic viscosity as a function of temperature is observed, typical behavior for the macromolecules dissolved in good solvents. An opposite behavior is depicted for PSU-b-PAO-b-PDMS triblock copolymer with a high content of PDMS, [[eta]] continuously increases with temperature, DCE behaves as a bad solvent for the sample 5. The homopolymer PSU (sample 9) presents the highest viscosity value because of a higher chain stifness and also because of the fact that the conformation of the macromolecules is more extended as a consequence of favorable polymer-solvent interactions. The PSU rigidity is due to relative inflexible and immobile phenyl and S[O.sub.2] groups and the toughness is attributed to the connection with ether oxygen [2, 24]. An introduction of more flexible blocks into the main backbone determines a decrease of the intrinsic viscosity (sample 6).

The discontinuities observed around 55[degrees]C in the viscosity-temperature plots for all PSU containing samples can be attributed to conformational transitions exhibited by the macromolecular chains, as it was previously observed for diblock copolymers [11]. Bellow the transition temperature, the triblock copolymers adopt a more or less segregated conformation, depending on their composition; by assuming that the number of heterocontacts is very small, the different polymer blocks behave as homopolymer chains. Above 55[degrees]C, the copolymer chains adopt a pseudo-Gaussian conformation, a random structure taking into account some overlaps between different types of blocks and thus heterocontacts between unlike segments are possible (Fig. 5). For high PDMS content (samples 4 and 5), especially for a molar content of 11% PSU in the copolymer, the viscosity increase is significant at high temperatures, suggesting a more segregated conformation at low temperatures when the unlike segments are rarely interacting. The viscosity-temperature dependence for the sample 5 suggests a bad solvent behavior of DCE for high PDMS content.

The spherical biphasic morphologies were revealed by TEM studies of the di- and triblock copolymers in solid state (Fig. 6). The morphology patterns within the block copolymers depend on the soft segments lengths. For low PDMS content (samples 1-3), TEM images revealed a biphasic morphology with the spherical micelles of hundreds of angstroms, formed by core of insoluble PDMS blocks and shell of PAO. The micelles are surrounded by PSU blocks having an extended conformation. The PDMS and PAO spheres with the size of 200-1000 [Angstrom] are dispersed in the continuous phase formed by the PSU blocks (Fig. 6a and b). By increasing the PDMS content (samples 4, 5), a lamellar structure appears (Fig. 6c), including the spherical formations.

CONCLUSIONS

The introduction of flexible segments into polysulfone (PSU) macromolecular chains determines a decrease of the viscosity and relative absorbance. The conformational transition depicted for PSU around 55[degrees]C was also observed for block copolymers of PSU with poly(alkylene oxide) (PAO) and the phenomenon is better evidenced for increasing content of poly(dimethylsiloxane) (PDMS). The changes in the macromolecular dimensions or UV absorption spectra of PSU-b-PAO-b-PDMS with low PDMS content by increasing the temperature were attributed to the transitions from a segregated conformation to a Gaussian one because of the enhanced interactions between unlike segments. Moreover, each type of segment interacts differently with the polar solvent. 1,2-Dichlorethane (DCE) is a good solvent for PSU and PAO and by increasing the temperature their conformation is more open. PDMS adopts a segregated conformation in DCE because of the preferential polymer-polymer interactions. The balance between these interactions is dictated by the copolymer composition and temperature and it has a major influence on the conformational characteristics. For low PDMS content, spherical micelles are formed by insoluble PDMS blocks surrounded by PAO blocks and dispersed in the continuous phase formed by the PSU blocks. For high PDMS content, a segregated lamellar conformation with a limited number of heterocontacts determines low viscosity values, even the block length increases.

ACKNOWLEDGMENTS

This work was supported by the grant Polymer Materials with Smart Properties (PN-II-ID-PCE-2011-3-0199) financed by the Romanian National Authority for Scientific Research, CNCS-UEFISCDI.

REFERENCES

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Maria Bercea, Anton Airinei, Viorica Hamciuc

"Petru Poni" Institute of Macromolecular Chemistry, 41-A Grigore Ghica Voda Alley, Iasi 700487, Romania

Correspondence to: M. Bercea; e-mail: bercea@icmpp.ro

This paper is dedicated to the memory of Dr. Aurelia Ioanid (1948-2011).

DOI 10.1002/pen.24400

TABLE 1. Characteristics of block copolymer samples
(PSU-b-PAO-h-PDMS) and precursors used in their
synthesis: PSU, PAO, PSU-fc-PAO, and PDMS.

                        Siloxane (a)   PAO (a)   PSU (a)
Sample cod                (% mol)      (% mol)   (% mol)


1 (PSU-h-PAO-h-PDMS)         18          57        25
2 (PSU-b-PAO-b-PDMS)         29          57        14
3 (PSU-h-PAO-h-PDMS)         43          43        14
4 (PSU-h-PAO-h-PDMS)         66          20        14
5 (PSU-b-PAO-b-PDMS)         66          23        11
6 (PSU-h-PAO)                --          50        50
7 (PDMS)                    100          --        --
8 (PAO)                      --          100       --
9 (PSU)                      0           --        100

                        [M.sub.w] x   [M.sub.n] x
                        [10.sup.-3]   [10.sup.-3]   [M.sub.w/
Sample cod                (g/mol)       (g/mol)     [M.sub.n]

1 (PSU-h-PAO-h-PDMS)       20.45       8.93 (b)       2.29
2 (PSU-b-PAO-b-PDMS)       15.20       7.15 (b)       2.12
3 (PSU-h-PAO-h-PDMS)       24.80       9.67 (b)       2.56
4 (PSU-h-PAO-h-PDMS)       26.20       11.50 (b)      2.28
5 (PSU-b-PAO-b-PDMS)       36.00       10.80 (b)      3.33
6 (PSU-h-PAO)              29.64       9.50 (b)       3.12
7 (PDMS)                    --         11.70 (c)       --
8 (PAO)                     --         2.20 (c)        --
9 (PSU)                    24.57       7.80 (c)       3.15

(a)(1) H-NMR spectra.

(b) GPC data.

(c) End groups analysis.


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Author:Bercea, Maria; Airinei, Anton; Hamciuc, Viorica
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Date:Jan 1, 2017
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