S. Fakirov

On the relationship between microhardness and glass transition temperature of some amorphous polymers

On the basis of microhardness (H) data measured at room temperature only for a number of polymers in the glassy state, a linear correlation between H and the glass transition temperature Tg has been found (H = 1.97Tg − 571). By means of this relationship, the deviation of the H values from the additivity law for some multicomponent and/or multiphase polymeric systems can be accounted for. The latter usually contains a liquidlike soft component and/or phase with Tg below room temperature. A completely different deformation mechanism in comparison to systems with Tg above room temperature is invoked. A novel expression for the hardness of polymers in terms of crystallinity of the single components and/or phases, the Tg values, and the mass fraction of each component is proposed. This expression permits the calculation of (i) the room-temperature H value of amorphous polymers, mainly containing single bonds in the main chain, provided Tg is known, and of (ii) the contribution of the soft liquidlike components (phases) to the hardness of the entire multiphase system.

Mechanical properties and transition temperatures of cross-linked oriented gelatin. 1. Static and dynamic mechanical properties of cross-linked gelatin

This study is an extension of previous work on cellulosics [(1994)Colloid Polym Sci 272: 284, 393] that showed that unusually good mechanical properties can be obtained by drying a swollen network of semirigid chains in a state of strain. This novel approach is applied in this investigation to gelatin, because of its attractive environmental characteristics but poor mechanical properties in the unmodified form. Since drawing of non-crosslinked gelatin is not practical, crosslinking by formaldehyde was used, followed by swelling, drawing and drying at fixed length. Mechanical tests were performed in static and dynamic modes. In this way improvements of Youngs modulusE, and stress at breakσb were determined as a function of gelatin concentration during drying. An increase inE andσb up to 2–3 times, and in the dynamic modulusE′ up to 6 times, was obtained when the draw ratio λ reached 4–5, after whichE, E′, andσb were found to decrease. Such behavior is explained by the highest orientation being achieved at λ=4–5, as proved by x-ray analysis. At λ=10–20 the orientation is lost due to relaxation of chain segments, which is preceded by partial destroying of the network structure (chemical and physical), possibly via chain scission, but probably mostly by the pulling out of chains from crystallites. In any case, the mechanical properties become poor again.The improvements reported above were referred to the undrawn but crosslinked gelatin. Compared to the starting isotropic non-crosslinked material, the improvement is slightly higher. The observation that the improvements are less than those obtained for the cellulosics is explained by the coexistence of interpenetrating chemical and physical networks, which is typical of gelatin. This structural feature drastically reduces the orientability of the chains and the improvements that can be expected in the mechanical properties.

Structure development in PET/PA6 microfibrillar-reinforced composites as revealed by revealed by microhardness

Homopolymer poly(ethylene terephthalate) (PET) and nylon-6 (PA6) and a blend (1 : 1 by weight) of these polymers, were extruded as strips and ultraquenched from the melt. After zone drawing and additional annealing at temperatures, Ta, of 220 or 240 °C for 5 or 25 h in vacuum, the samples were studied by scanning electron microscopy (SEM), wide-angle X-ray scattering, solubility and microhardness, H, tests. In conformity with previous studies of the same system, the present SEM observations show that mechanical drawing results in the formation of a highly oriented fibrillar structure of PET which is preserved even after annealing above the melting point of PA6. Furthermore, raising of both annealing temperature and duration up to 240 °C and 25 h, respectively, results in a strong decrease of the solubility of the PA6 fraction in formic acid (five-fold). This is attributed to intensive chemical interactions between components, drastically improving the adhesion between matrix and reinforcing microfibrils. From the dependence of H on degree of crystallinity, wc, the hardness values for completely amorphous, Ha, and fully crystalline, Hc, neat homopolymers were extrapolated (HaPET = 128 MPa, HcPET = 294 MPa, HaPA = 52 MPa and HcPA = 283 MPa). Using these values and applying the additive law, the H-value of the microfibrils is derived. The high value obtained for PET fibrils (360 MPa) is explained by the peculiarity in the structure formation of these microfibrils. The effect of crystal size on the formation of H is also discussed. The H-value of infinite large PA6 crystals is derived to be H∞ = 460 MPa. It is shown that the type and extent of the mutual dispersion of the components, as well as the adhesion between them, are important factors for the proper applicability of the additive law.

Strain-induced β-α polymorphic transition in iPP as revealed by microhardness

The microhardness (H) technique was used for characterization of the β-α polymorphic transition in isotactic polypropylene (iPP). For this purpose the microhardness in the damage zone of a tensile loaded deeply edge-notched (DEN-T) β-iPP specimen was mapped. Mapping of H was performed, both along the loading direction (central) and close to the shorter fracture edge. Around half-length of the plastic zone a sharp increase of the H values in both cases was observed. The H increase is related to the β → α polymorphic transition. Microvoid formation in the central part results in lower H values. However for the edge zone close to the top of the fracture surface unusually high H values (around 200 MPa) are obtained. The latter are explained in terms of the formation of microfibrils due to crazing during deformation which are characterized by very high molecular orientation as reported from X-ray analysis.

Sequential reordering in condensation copolymers, 4. Crystallization-induced sequential reordering in poly(ethylene terephthalate)/polycarbonate copolymers as revealed by the behavior of the amorphous phases†

An equimolar blend of poly(ethylene terephthalate) (PET) c and bisphenol-A-polycarbonate (PC) d is studied by dynamic-mechanical thermal analysis (DMTA) and X-ray scattering after thermal treatment that enables transesterification. As demonstrated by wide-angle X-ray scattering (WAXS) measurements, prolonged thermal treatment at 280°C gives rise to a copolymer that no longer reveals melting or crystallization. In accordance with previous reports, this effect is attributed to the formation of a random copolymer. Additional annealing of such samples below the melting temperature of PET results in restoration of the crystallization ability. This effect is explained by crystallization-induced sequential reordering from random to block copolymer by means of transreactions which closes the cycle of transformations from two homopolymers via block- and random copolymer back to a block copolymer. The behavior of the amorphous phases is studied by means of DMTA demonstrating that their glass transition temperatures T g s vary in accordance with the crystallinity changes. The random copolymer is characterized by a more or less homogeneous amorphous phase. In contrast to this, the mechanical mixture and the two block copolymers (the initial and that with the restored blocky structure) show DMTA peaks of two amorphous phases, clearly separated and with distinct individual Tgs. Viscosity measurements also demonstrate that the random copolymer significantly differs in its viscosity as compared to all other samples. These results represent a further evidence for the effect of block restoration via crystallization-induced sequential reordering.

Microhardness of condensation polymers and copolymers. 1. Coreactive blends of polyethylene terephthalate and polycarbonates

Abstract The microhardness of coreactive blends of polyethylene terephthalate (PET) and bisphenol A polycarbonate (PC) was investigated over the whole range of compositions. The occurrence of one single glass transition temperature (T g) step in the differential scanning calorimetry (DSC) curves indicated that intensive chemical interactions had taken place during melt blending, resulting in formation of copolycondensates with dominating random sequential order. The parallel decrease of microhardness (H) and of Tg with increasing PET content in the blends has been ascribed to the formation of new copolymer molecules enriched in the component characterized by lower H and T g values. It is emphasized that such noncrystallizable copolymers offer the possibility to evaluate the intrinsic contribution of the repeating units to the H and T g characteristics of copolymers with various compositions and sequential orders.

Sequence length determination in poly(ethylene terephthalate)–bisphenol-A polycarbonate random copolymers by application of selective degradation

Poly(ethylene terephthalate) (PET) and bisphenol-A polycarbonate (PC) are melt-mixed in equimolar ratios under various conditions to get a series of PET–PC copolymers. Samples from each copolymer are characterized by differential scanning calorimetry, 1H and 13C nuclear magnetic resonance (NMR), size exclusion chromatography (SEC), and polarizing light microscopy. The lengths of the PET sequences are determined in different copolymer samples by NMR sequential analysis before and after removal of the PC segments by selective degradation. In the former case, rather unusual results are obtained, suggesting predominant alternating order of single PET and PC repeating units. After selective elimination of the PC units, however, the NMR techniques show evidence of consecutively bonded dyads or triads of PET and PC units, which corresponds to the theoretical values in random copolymers obeying the statistics of Bernoulli. Considering the 1H-NMR and SEC results after selective elimination of the PC sequences, a possible structure of the residual PET containing segments is proposed for the first time. It is concluded that in the PET/PC copolymers studied, when sequence distribution approaches the random one, determination of the PET block lengths after elimination of the PC sequences is more reliable as compared to the cases when selective degradation is not applied.

New aspects of thermal treatment effects on gelatin films studied by microhardness

Microhardness measurements were carried out using various treatment cycles, including heating and cooling during different treatment times at high temperatures. Two possible processes to explain the observed increase in the microhardness are proposed, namely crystallisation and crosslinking. In order to distinguish between these two alternatives, differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and swelling kinetics measurements were performed. The observed decrease of crystallinity (from DSC and WAXS measurements) as well as the decrease of swelling ability with increasing temperature and duration of thermal treatment are in favour of the occurrence of crosslinking reactions during thermal treatment. It is suggested that the crosslinking, as a result of additional intra- and intermolecular condensation processes, leads to a denser chain packing in the amorphous gelatin and consequently to higher microhardness values.

Structures and mechanical properties of zone‐drawn–zone‐annealed blends of cocrystallizing poly(butylene terephthalate) and a poly(ether ester)

Poly(butylene terephthalate) (PBT) and a poly(ether ester) (PEE) based on PBT and poly(ethylene glycol) were melt-blended and extruded as films with quenching. They were then zone-drawn (ZD) and zone-annealed (ZA) at various stresses (between 10 and 50 MPa) at temperatures of 160 and 190° C. The goal was to improve their mechanical properties relative to those of the same blend, but cold-drawn (X = 5) and isothermally annealed with fixed ends at the same temperatures for 6 h. All samples were characterized by DSC, WAXS, SAXS, and static mechanical property measurements. In contrast to the isothermally annealed samples, the zone-drawn and zone-annealed ones exhibit one population of crystallites arising from the homo-PBT, as demonstrated by the DSC and SAXS measurements. In addition, however, the WAXS photographic patterns indicate that zone annealing at 190°C results in isotropization of crystallites originating from the PEE, resulting in the formation of a microfibrillar-reinforced composite. It is assumed that some of the isotropic crystallization occurs on preexisting homo-PBT crystallites, i.e., a partial cocrystallization occurs, improving the adhesion between the components of the blend. The structural features created in the zone-drawn-zone-annealed materials result in higher values of the Youngs modulus and tensile strength in comparison to the materials receiving the simple isothermal treatment (1,200 vs. 480 MPa and 213 vs. 113 MPa, respectively).

Gelatin films with very high surface hardness

Microhardness ( H ) of gelatin dry cast films was measured in the 100-250°C range. A strong H increase with temperature from 330 MPa up to 450 MPa was observed in the first cycle. The H values reached in the subsequent cycles are higher than those obtained in the preceding ones, surpassing the hardness of all commercial synthetic polymers and soft metals. Results are discussed in terms of crosslinking between side-chain reactive groups.