Physicochemical characterization of alloy of polymide with varying degree of crosslinking through diisocyanates.

INTRODUCTION

Research in the area of polymeric blends, alloys, composites, and interpenetrating network systems has been very active in both academic and industrial fields. The value addition of commercial polymers (both commodity and speciality grades) and avenues for fundamental research in these emerging fields have culminated in tailoring of the properties of the polymers to meet very specific technological requirements. Polymer blends and alloys are relatively economical to produce, and generally cause a lower technical risk than developing a new polymer or polymeric compositions (1). Alloys are synergistic polymer combinations with real property advantages derived from a high level of compatibility between components. Alloys exhibit strong intermolecular forces and form single phase systems with one glass transition temperature. The discovery of new polymeric alloys by blending commercially available polymers is an exciting trend in material development. The ability to tailor existing materials to a new/unique set of property/performance/price specifications through the combination of materials is the key reason for this growth. Polyimides, the polymers containing cyclic imide groups in the main macromolecular chain, have emerged as potential high temperature resistant materials finding myriad applications in frontier technologies like aerospace, microelectronics, and advanced composites (2). The blending or alloying in polyimide fields is not reported, which could be attributed to their intractable character. In multiphase polyimide systems, molecular composites (3) and polyimide ceramers (4) have recently been reported.

As a consequence of extensive studies (5-10) in our laboratory involving the modifications of polyimides by metal salts and organic crosslinkers, a new approach for the synthesis of polyimide blends/alloys has emerged. In this new concept, a blend of polyamic acid with a varying degree of crosslinking is in-situ produced, and alloys of polyimides are produced from thermal cyclization. A very lightly crosslinked polyamic acid is produced from the reaction of a very highly functionalized polyamic acid with hypotheticaUy calculated concentration of crosslinker (either organic or metal salts). A hypothetical concentration has been calculated based on the concept of critical branching [TABULAR DATA FOR TABLE 1 OMITTED] coefficient, and crosslinked polyamic acid component is produced from the existing polyamic acid molecules. The blend of the polyamic acids thus obtained has been thermally imidized to result in polyimide alloys. The synergistic improvement in thermal (9), mechanical and water uptake characteristics (7) has already been demonstrated, and an attempt to elucidate the mechanism responsible for these properties has been made and also has been currently undertaken to strengthen this new concept of making alloys and blends of polyimides. In continuation of these investigations, a new series of polyimide alloys using this concept is being prepared wherein the varying degree of crosslinking or branching is introduced via diisocyanate reaction. This communication discusses with the synthesis, physico-chemical characterization, and mechanical properties evaluation so far conducted in our laboratory of polyimide-methylene bis-phenyl isocyanate alloys and blends.

EXPERIMENTAL

Materials and Instruments Used

Polyamic acid (PAA) - a condensation product of pyromellitic dianhydride (PMDA) and oxy dianiline (ODA) was procured from M/s ABR Organics Ltd., Hyderabad, India; this product is marketed as ABRON S-10, which is a 10% solution in dimethyl acetamide (DMAC). Here, the intrinsic viscosity of PAA is given as 1.5 dL/g by the manufacturer, but its intrinsic viscosity was determined to be 0.95 dL/g in the laboratory. The PAA was kept in an airtight container stored at low temperature (-2 to + 3 [degrees] C). The crosslinker methylene bis(4-phenyl isocyanate) or MDI was obtained from E. Merck, Germany, and was used as received. The solvent DMAC, reagent grade (b.p.162 [degrees] C to 166 [degrees] C), was supplied by CDH laboratories, Mumbai, India. DMAC was purified by distillation in the presence of [P.sub.2][O.sub.5] and the fraction between 162 [degrees] C and 166 [degrees] C was collected and was kept in a closed container. The intrinsic viscosity of the PAA solution was determined using an Ubbelohde viscometer with the following relationship of intrinsic viscosity with molecular mass (M). The viscosity average molecular mass of the PAA polymer was calculated using the following relationship between intrinsic viscosity [[Eta]] and the average molar mass.

[Eta] = K [M.sup.[Alpha]] (1)

where K and [Alpha] are the Mark-Kuhn-Houwink constants. The values of K and [Alpha] for this study have been taken as 2.3 x [10.sup.-4] and 0.78, respectively, as reported elsewhere (11, 12).

The degree of imidization of all samples was determined by the IR spectroscopic technique. Spectra of PAA/polyimide films were recorded using Perkin Elmer's FTIR spectrophotometer (model 1720 X). A band ratio method was adopted wherein the area of symmetric carbonyl stretch at 1776 [cm.sup.-1] was ratioed with the area of reference aromatic vibration at 1012 [cm.sup.-1] (13). DMAC was chosen as the solvent because it does not interfere in the IR spectra of Imide and aromatic peaks of polyimide (PI). The solvent content was determined by the weight difference method using an electronic digital balance with intervals of 0.001 mg. Mechanical properties such as tensile strength, elongation, and Young's modulus were determined according to ASTM test method D 882. A Instron Universal Testing Machine (model 4202) was used and a crosshead speed of 0.5 mm/min was adopted as per test method. The average of the five measurements were taken.

Synthesis/Preparation

A calculated quantity of MDI in DMAC was mixed in PAA solution at different concentration levels so as to result in PI-MDI films of 25-micrometer thickness with varying concentrations of MDI, viz., 1.54 x [10.sup.-2], 1.54 x [10.sup.-3], 1.54 x [10.sup.-4], 1.54 x [10.sup.-5], and 1.54 x [10.sup.-6] mol/L of polyimide, designated as PI-MDI-2, PI-MDI-3, PI-MDI-4, PI-MDI-5, and PI-MDI-6, respectively. [TABULAR DATA FOR TABLE 2 OMITTED] The neat PI (i.e. without MDI) is designated as PI-1. The quantity of MDI solution in DMAC should be such that its addition to PAA solution should not substantially affect solvent concentration. MDI and PAA solutions were mixed thoroughly and spread over a glass plate by means of a doctor's blade. For thermal imidization and as well as solvent removal, the samples were heated at different curing temperatures (i.e. 50 [degrees], 100 [degrees], 150 [degrees], 200 [degrees], 250 [degrees], 300 [degrees], and 350 [degrees] C) for 2 h. The higher cure temperature (i.e. 100 [degrees] to 350 [degrees] C) is, however, accomplished by slow heating rates, and the temperature is kept constant for 10 min at 50 [degrees], 100 [degrees], 150 [degrees], 200 [degrees], 250 [degrees] or 30 [degrees] C, respectively, at ultimate cure temperature of 50 [degrees], 100 [degrees], 150 [degrees], 200 [degrees], 250 [degrees]m 300 [degrees], and 350 [degrees] C. These temperature-time profiles have been calculated so as to prevent micro "blow holes" in the polyimide films. The curing temperature-time profiles were designated as PROF-50, PROF-100, PROF-150, PROF-200, PROF-250, PROF-300, and PROF-350, respectively, and wherein numbers (i.e. 50, 100, 150, 200, 250, 300, and 350) indicate the ultimate cure temperature. The samples were cooled to room temperature after curing at all the designated temperature-time profiles and removed from the glass plates.

RESULTS AND DISCUSSION

Polyamic acid (PAA) could be considered as polyfunctional polymer in terms of carboxyl functionality [TABULAR DATA FOR TABLE 3 OMITTED] as they react with the isocyanate functional groups of the MDI. The established reaction scheme (14) between isocyanate groups and the carboxylic groups of PAA is represented in Fig. 1.

The carboxyl functionality of PAA can vary approximately from 200 to 700 depending upon its molecular mass. Any material reactive towards PAA and having the functionality of two or more can crosslink PAA. The average molecular mass of PAA used here was calculated from Eq 1 wherein the value of [[Eta]] was 0.95 dL/g. The functionality shall, therefore, be 218. The formation of infinite networks (15) in such polyfunctional branching units will be governed by the critical branching coefficient (i.e. [[Alpha].sub.c]) by its simple correlation as given in Eq 2.

[[Alpha].sub.c] = 1/(f - 1) (2)

where 'f' is the functionality of the branching unit like PAA. This indirectly implies that a very low concentration of crosslinker like bifunctional MDI will introduce adequate crosslinking in PAA. In order to quantitatively assess the effect of MDI incorporation in polyimide, a hypothetical concentration of bifunctional MDI was used and the same has been termed "critical crosslinker concentration" (CCC). The CCC has been visualized as the concentration of MDI (in mol/L of PI) required to introduce single crosslinkage between two branching units, i.e., polyamic acid molecules. The CCC was calculated to be 1.54 x [10.sup.-2] moles of crosslinker in a liter of polyimide (and not polyamic acid). The incorporation of MDI crosslinker was affected by the concentration equal or lower to CCC. It can be safely presumed that the crosslinker concentration lower than CCC will result in two components, i.e., one having very low crosslinked structure (or mostly branched structure) and the other one having neither crosslinking nor branched structure. Since the crosslinking or branching occurs only at lower temperatures (i.e. below the temperatures where thermal cyclization commences), the resultant blend will have two components consisting of branched or lightly crosslinked and uncrosslinked (or unbranched) PAA. The mass fraction of crosslinked and uncrosslinked molecules will vary depending upon the concentration of crosslinker, and their fractions in the blend can be calculated. The mole fraction of the crosslinked PAA in a blend has been theoretically calculated presuming the crosslinked molecules are a network of polyimide with single crosslinkage. The mole fractions of crosslinked to uncrosslinked components in a blend with different levels of crosslinkers are given in Table 1. The PAA blends were prepared wherein the crosslinker concentration was varied from CCC and up to four orders lower than CCC. These polyimide blends in film form were produced by thermal imidization of PAA blends. The results of physicochemical and mechanical properties of the end products, i.e., polyimide blends, were determined. The results of these properties for PI blends were compared with a neat PI blend in order to assess the effect of the blending at different crosslinker concentrations.

As mentioned earlier, the crosslinking reaction takes place at polyamic acid stage itself resulting in blends of polyamic acid. The thermal cyclization of these polyamic acid blends at different curing conditions, i.e., temperature-time profiles, was studied with respect to the degree of imidization and the residual solvent content. The results of the degree of imidization at different temperature-time profiles and for various crosslinker concentrations are presented in Table 2. It was evident that a substantial degree of imidization was achieved only above PROF-200 in neat samples. It was observed that accomplishing a degree of imidization was sluggish with an increase of crosslinker concentration. This phenomenon was more pronounced at low cure temperatures. As an illustration, the difference in the degree of imidization at PROF-50 cure was 32% against 9% at PROF-350 cure. The low degree of imidization at higher crosslinker level could be attributed to the higher [T.sub.g] in the crosslinked region, which would have hindered the segmental molecular motion, which in turn would have prevented the cyclization reaction. The effect of solvent, i.e., DMAC or DMF, on the thermal cyclization of PAA was studied in detail previously (16-19). The solvent was found to associate with the carboxyl groups of PAA through hydrogen bonding (20). These associations are destroyed during imide ring formation, and solvent is released. It was reported that evolution of water from imidization and release of solvent occur practically simultaneously.

Table 4. Young's Modulus of PI-MDI Blends With Varying Crosslinker
Concentrations and Cured at PROF-50 and PROF-300.

Sample                    For PROF-50                For PROF-300
Designation(*)               (MPa)                      (MPa)

PI-1                          333                        2500
PI-MDI-6                     1400                         875
PI-MDI-5                     1000                        3000
PI-MDI-4                      400                        1190
PI-MDI-3                      700                        1667
PI-MDI-2                      333                         591

Note: The details of sample designation and PROF-50 and PROF-300 are
same as in Tables 2 and 3.

In light of these observations and the studies conducted by Swarnkar et al. (7), it became essential to study the residual solvent content for PI-MDI blends cured at different temperature-time profiles. These results are presented in Table 3. The solvent content at PROF-50 was found to be 49.1% in the case of neat, and the same is reduced to a slightly lower level, 45%, at CCC. Similarly, at PROF-100 cure, the residual solvent content was varied between 26.2% to 27% for PI-MDI-2 and PI-1, respectively. These observations are more or less similar to those of the previous studies (5-10). Hence, the residual solvent at PROF-50 amounts to a 4/1 complex of DMAC solvent and PAA, and at PROF-100 it amounts to a 2/1 complex as shown in Fig. 2. At the subsequent cure temperature, the hydrogen bonded DMAC slowly dissociates from the polymer, facilitating the imidization process. The DMAC-PAA interaction resulting in these two type of complexes and subsequent decomplexation at higher temperatures are described in the reaction process as shown in Fig. 2. A 4/1 complex of DMAC with monomeric unit of PAA should theoretically have 45.5% of DMAC hydrogen bonded (or complexed) with the PAA, as shown in Fig. 2. As a matter of fact, the residual solvent content was found between 45% and 50% at PROF-50 cure for all the samples, and this indicated that the 4/1 complex did exist along with free solvent. It was observed that the residual solvent content at PROF-100 cure was 25.7% to 27.5%, which is a little lower than the theoretically calculated value of 29.4% for the 2/1 complex. These observations are quite in agreement with the extensive studies conducted on the model compounds by Brekner and Feger (21) and with those of Swarnkar et al. (7). The lower solvent content with increased crosslinker concentration could be rationalized with the relationship between the crosslink density of the polymer network with its swelling by a solvent as given by the Flory-Rehner equation (22, 23). Within the experimental error, the residual solvent content was found to be lower at higher crosslinker concentration. The residual solvent content data have established that the PI-MDI blend will have less solvent after curing, which is a very attractive property.

The mechanical properties are dependent on the degree of crosslinking and the chain length of the primary macromolecules. As mentioned in the Introduction, only a few studies have been conducted for the crosslinking of condensation-polyimides, whereas in the case of addition types, this aspect has been studied extensively. The enhancement of the tensile strength and tensile modulus was reported in the case of metal salt-modified PI wherein the enhancement in the tensile properties was attributed to the inadvertent crosslinking introduced (24). The PI alloys prepared by the method suggested here have shown overall improvement in the mechanical properties (7, 8); however, these alloys were prepared with metal salts or complexes. In the present study, MDI, an organic crosslinker, has been used, so the relation of mechanical properties with the crosslinker concentrations assumes significance. The three parameters-tensile strength, elongation at maximum load, and Young's modulus - have been considered to evaluate the effect of the crosslinker concentration. Figure 3 gives the tensile strength at different concentrations when cured at designated temperature-time profiles. The tensile strength shows a general increase with an increase in the cure temperature. The PI-MDI blends having a concentration of crosslinker from [10.sup.-6] mol/L to [10.sup.-3] mol/L have demonstrated improved tensile strength at all cure temperatures. Nevertheless, the extent of improvement in tensile strength was visibly higher for blends cured at lower temperatures than their counterparts cured at high temperature. The blend PI-MDI-5 shows the peak improvement over other blends having both lower and higher concentration of crosslinker. The PI-MDI-5 blend (i.e. [10.sup.-5] mol/L), when cured at PROF-50, shows a tensile strength of 60.55 MPa, which rises to 102.60 MPa at PROF-350 cure. More or less, a similar trend was observed for elongation values as given in Fig. 4. The synergism in the elongation was observed in the alloys between [10.sup.-6]mol/L and [10.sup.-4] mol/L crosslinker concentration, and invariably the elongation for the sample having 1.54 x [10.sup.-2] mol/L (i.e. CCC) was lower than for other alloys and for the neat.

From the mechanical properties point of view, the modulus of elasticity is of great importance. It is essential that instead of studying only the initial modulus values, one should look at the whole stress-strain response. A comparison of these responses is made at PROF-50 and PROF-300. The results of PROF-350 are important. However, these have not been considered because of the wide scatter in the values. At 50 [degrees] C or PROF-50, it is expected that crosslinking is introduced, and both components of the blends are generated. The stress-strain curves at PROF-50 and PROF-300 are shown in Figs. 5 and 6, respectively. A preliminary examination of the stress-strain response curve indicates that at 50 [degrees] C, the Young's modulus values are very low (in the range of 300 to 400 MPa for PI-1 and PI-MDI-2, whereas in the blend compositions, i.e., PIMDI-3, PI-MDI-4, PI-MDI-5, and PI-MDI-6, the value ranges from 400 to 1400 MPa, and the highest, 1400 MPa, was for PI-MDI-6, as shown in Table 4. In PROF-300, the initial modulus values do not show a uniform trend. PI-MDI-2, where the crosslinker concentration is equal to the critical crosslinker concentration, shows the lowest modulus. The PI-1 sample has an initial modulus of 2500 MPa whereas the magnitude of the modulus for PI-MDI-5 is 3000 MPa. The Young's modulus value of 875 MPa for PI-MDI-6 cured at PROF-300 appears to be quite low, and this could not be explained. As the magnitude of the modulus of elasticity is indicative of the main type of a inter-atomic linkages that govern the mechanical properties of the polymers, a higher magnitude of the modulus for blend composition indicates the occurrence of chemical bonds, i.e., crosslinking. The amorphous and the crystalline nature also play a role in determining the magnitude of the Young's modulus, and it is very likely that the small amount of crosslinking may change the amorphous character of the polyimide films. Despite the scatter in the mechanical properties, i.e., tensile strength, elongation, and Young's modulus, cured blends have unmistakably exhibited the synergism in the blend PI-MDI-5 (1.54 X [10.sup.-5]mol/L). The realization of synergism in the properties is seen by comparing the properties of the blends with those of unmodified polyimide (i.e. neat). The improvement in the properties or synergism by microlevel modification has not been reported for polyimides. The studies conducted here are only of an indicative nature, and more exhaustive investigations are under way. The structure property correlation will be explored in the near future.

CONCLUSION

An alloy of polypyromellitimide was synthesized by thermal imidization of the blend consisting of crosslinked and uncrosslinked PAA components. The variation in the degree of crosslinking was introduced through the amic acid isocyanate reaction, and a new concept emerged from the theory of critical branching coefficient. The degree of imidization and residual solvent were determined at seven temperature-time profiles ranging between 50 [degrees] C and 350 [degrees] C. The spectrum of the mechanical properties obtained by varying the crosslinker concentration and temperature-time profiles was presented and the synergism in the mechanical properties in the blend compositions was established.

ACKNOWLEDGMENTS

The authors express their gratitude to Dr. K. V. C. Rao, vice chairman, ABR Organics Ltd., Hyderabad, for his help and advice. The mechanical properties determination by the staff of the Analytical Division, Space Centre, Trivandrum, India, is gratefully acknowledged. The authors thank Mr. Rajendra Dubey for computational and secretarial help.

ABBREVIATIONS

CCC = Critical Crosslinker Concentration.

MDI = Methylene bis (4-phenyl isocyanate).

PAA = Polyamic acid.

PMDA = Pyromellitic dianhydride.

ODA = Oxy dianiline.

DMAC = Dimethyl acetamide.

PI = Polyimide.

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