Peter P. Huo

Effects of thermal history on the rigid amorphous phase in poly(phenylene sulfide)

The rigid amorphous phase of semicrystalline poly(phenylene sulfide) (PPS) has been studied as a function of thermal history using scanning calorimetry, dielectric relaxation, density, and small-angle x-ray scattering (SAXS). Based on the new heat of fusion of perfect crystalline PPS, which is 26.7±0.8 cal/gram, the weight fraction of rigid amorphous phase is shown to be nearly twice as large as previously reported [1]. The mass fraction of the rigid amorphous phase ranges from 0.24 to 0.42 and is dependent upon thermal treatment. We have taken the approach of assuming a three-phase model for the morphology of semicrystalline PPS consisting of crystalline lamellae, mobile amorphous, and rigid amorphous components. Using this three-phase model, we determine that the average density of the rigid amorphous fraction is 1.325 g/cc, which is slightly larger than the density of the mobile amorphous phase fraction and was insensitive to thermal history. From the SAXS long period, the layer thicknesses of the mobile amorphous phase, rigid amorphous phase, and crystal lamellae were estimated. Only the lamellar thickness shows a systematic variation with thermal history, increasing with melt or cold crystallization temperature, or with decreasing cooling rate.

NEW-TPI thermoplastic polyimide: thermal analysis and small-angle X-ray scattering

Isothermal cold crystallization of the thermoplastic polyimide NEW-TPI has been investigated from 300 to 360°C. The fastest crystallization took place at 330°C where the time to maximum exothermic heat flow was 148 s. A single Avrami exponent could be used to describe the bulk crystallization kinetics for degrees of conversion from 0.02 up to 0.95, indicating that most crystallization takes place at the growth front by a single mechanism, nearly up to the completion of crystallization. The Avrami exponent is 3.5 for Tc < 330°C, and decreases as Tc increases. A regime II to III transition is indicated by a slope change seen in ln(lt12) + UR(T−T∞) vs. 1TΔTf. These results indicate that NEW-TPI cold crystallization can be modelled according to heterogeneous nucleation and three-dimensional crystal growth. From Tm vs. Tc analysis, the infinite-crystal melting point of NEW-TPI is estimated to be 400°C. With regard to the glass transition, Tg was affected by cold crystallization in a systematic but minor way. The heat-capacity increment of semicrystalline NEW-TPI at Tg shows a small negative deviation from that predicted on the basis of the degree of crystallinity. The absence of secondary crystallization processes has been used to explain the different glass transition behaviour and small negative heat-capacity deviation in NEW-TPI. Real-time small-angle X-ray scattering (SAXS) was used to monitor the structure change as a function of cold crystallization time at 300°C. Systematic change of long period, lamellar thickness, invariant and volume fraction of crystallinity were obtained from the one-dimensional electron density correlation function analysis. At Tc=300°C, lamellar thickness as a function of tc ranges from 37 to 52 A, or about 1.5–2.0 times the monomer repeat unit. The thickness of the interphase region was determined from SAXS to be about 17 A independent of cold crystallization time at 300°C.

NEW-TPI thermoplastic polyimide : dielectric and dynamic mechanical relaxation

Abstract The dielectric and dynamic mechanical relaxation behaviours of the thermoplastic polyimide, NEW-TPI, have been investigated from 150 to 350°C, which spans the glass transition region. Dynamic modulus at 1 Hz is about 2.2 GPa below the glass transition temperature, T g , decreasing to 0.02 GPa in amorphous NEW-TPI, and 0.15 GPa in semicrystalline NEW-TPI, above T g . Dielectric constant at 10 kHz is about 3.21 below T g , increasing to 3.44 in amorphous NEW-TPI, and 3.33 in semicrystalline NEW-TPI, above T g . Williams-Landel-Ferry (WLF) plots for amorphous and one representative semicrystalline NEW-TPI were constructed from thermal, dynamic mechanical and dielectric relaxation data but the data could not be fitted to a single master curve. Dielectric relaxation intensity, Δe = e s − e ∞ , was shown to be structure sensitive above T g . For both semicrystalline and amorphous NEW-TPI, the relaxation intensity decreases as temperature increases. This implies that Δe has the same temperature dependence for the semicrystalline sample compared to the quenched amorphous polymer. This trend is different from that observed in either poly(ether ether ketone) or poly(phenylene sulphide). Our results confirm thermal analysis of NEW-TPI and show that NEW-TPI has a very small amount of tightly bound, or rigid, amorphous material, which relaxes completely within a narrow temperature range just above the T g of the less tightly bound, or mobile, amorphous material.

Dielectric relaxation as a probe of interphase structure

Abstract The electrical and thermal properties of two high-performance polymers, PEEK, poly(etheretherketone), and NEW-TPI semicrystalline thermoplastic polyimide, are reviewed and compared in this work. Dielectric relaxation was used as a probe of the crystal/amorphous interphase and is shown to be sensitive to the interphase structure in the temperature range above the glass transition. The dielectric relaxation intensity is related to the number density of dipoles that are relaxed at a given temperature. For NEW-TPI, the constrained amorphous phase is completely relaxed within thirty degrees above T g , while for PEEK the interphase relaxes more gradually. Thermal analysis shows that PEEK contains a much larger fraction of constrained amorphous chains and a much smaller fraction of mobile amorphous chains, compared to NEW-TPI. PEEK crystallizes more rapidly above the glass transition, and a large fraction of PEEK crystallinity is attributed to secondary crystallization processes. In NEW-TPI, as a result of the increased chain stiffness, cold crystallization is slow and little or no crystals develop by secondary crystallization. These differences in crystallization behaviour of the two materials may be related to the differences in the formation of a rigid amorphous interphase.

13C n.m.r. study of phase heterogeneity in poly(phenylene sulphide)

Abstract Solid state 13C n.m.r. was used to study the phase heterogeneity of poly(phenylene sulphide), PPS, at room temperature. Relative fractions of crystal, rigid amorphous and liquid-like amorphous phases were determined from X-ray and thermal analyses. Samples containing very different fractions were examined using cross-polarization contact time and spin-lattice relaxation measurements. The optimum carbon/porton contact time for cross-polarization/magic angle spinning studies was found to be ∼ 1000 μs. Peak intensity of the protonated carbons was studied as a function of delay time. A pronounced amorphous halo exists in the n.m.r. spectrum of PPS films containing a large fraction of liquid-like amorphous phase, and the halo decreases significantly with increasing delay time. A very insignificant halo exists in the n.m.r. spectrum of PPS containing a large fraction of rigid amorphous phase. We conclude that this halo is from the most mobile, liquid-like amorphous phase. Using spin-lattice relaxation measurements, we studied the dynamic heterogeneous phase behaviour of PPS. Here we find that from the standpoint of spin-lattice relaxation time, T1, 100% amorphous PPS shows phase heterogeneity at room temperature. The longest T1 is ∼ 45 s, corresponding to a mass fraction of 0.57. In semicrystalline PPS, spin-lattice relaxation measurements show a much larger amount of the material possesses longer relaxation time. The T1 of semicrystalline PPS ranges from 150 to 200 s, corresponding to mass fractions of 0.91–0.94. This indicates that semicrystalline PPS is more nearly homogeneous: the crystals and most of the amorphous phase are very ‘rigid’ at temperatures far below the glass transition of PPS.

Characterization of Polymer Structure Using Real-Time X-ray Scattering

X-ray scattering is a powerful analytical tool for the non-destructive evaluation of crystallizable polymers and blends. Our group has been using real-time x-ray scattering to study thermal properties and microstructure development in semicrystalline polymers1–4. Here we describe our research on thermal expansion studies of poly(butylene terephthalate)1, PBT, using x-ray scattering for non-destructive property evaluation. X-ray scattering experiments were conducted at the Brookhaven National Synchrotron Light Source (NSLS).