Thermal stability and dynamic mechanical properties of acetal copolymers

By copolymerization of trioxane with 1,3-dioxacycloheptane (DOCH) or 1,3-dioxacyclooctane (DOCO) acetal copolymers were synthesized with comonomer units which, in contrast to customary 1,3-dioxolane/trioxane copolymers, consist of sequences with four or five methylene groups, respectively. In comparison with 1,3-dioxolane/trioxane copolymers these copolymers show a significantly higher thermal stability due to the larger sequences of thermically stable C—C-bonds. Torsion pendulum measurements show that the difference of the comonomer nature influences the storage modulus, G′, which is a measure for the rigidity of the material, and the loss tangent tan δ. Obviously, G′ is dependent on the crystallinity of the sample, which again is influenced by the kind and concentration of the incorporated comonomer. Also the peak locations of the α- and γ-relaxation indicate a direct connection to the comonomer nature and content in the different copolymers. The α-maxima are shifted to lower temperatures with increasing number of methylene groups as well as with growing comonomer content. However, the γ-maxima appear at higher temperatures with increasing comonomer content, but at lower temperatures with growing length of the comonomer unit. Additionally, branched acetal copolymers were synthesized by using comonomers in which the hydrogen atom at the 4-position of 1,3-dioxolane has been substituted by alkyl groups of different length (methyl to tetradecyl). In these copolymers the nature of comonomer neither improves the thermal stability of the copolymers nor influences the α- and γ-relaxation.     

Influence of structural parameters on the dynamic mechanical properties of polyacetals

The dynamic mechanical behaviour of various polyacetals was determined by means of torsion pendulum measurements. The products used had different structural parameters such as molecular weight distribution or comonomer nature and content, respectively; thus it was possible to investigate the influence of these parameters on the temperature dependence of the storage modulus G′, which is a measure for the rigidity of the material, and the loss tangent tan δ from -100 to 160°C. The storage moduli G′ are not influenced by different molecular weight distributions, but correlate with the crystallinity in the samples, which is, beside others, dependent on the concentration of the incorporated comonomer components. A decreasing crystallinity - i.e. an increasing comonomer content - is combined with a loss in rigidity. The distance between the peak location of the α-relaxation in the loss tangent curves and the melting points (differential scanning calorimetry) of the investigated acetal resins is nearly constant; thus the molecular motions causing the β-process take place in the crystalline regions. The α-process (around ?7°C) arises predominantly from molecular motions of the comonomer units, whereas the intensity of this relaxation can be correlated with the mobility of these units. The peak of the γ-relaxation is located around ?67°C and does not hardly shift despite of different structural parameters in the sample, whereas its intensity decreases with increasing both comonomer contents and contents of low-molecular-weight components. For unimodally distributed polyacetals with different comonomer contents, the intensity of the β-relaxation increases with increasing comonomer contents; simultaneously the intensities of the α- and γ-relaxatìon decrease.     

Blends of propylene-ethylene and propylene-1-butene random copolymers: I. Morphology and structure

Samples of propylene-ethylene (EP) and propylene-(1-butene) (BP) random copolymers with various comonomer content (2-3.1 wt% ethylene, 9.9 wt% 1-butene), were melt-mixed in Brabender internal mixer at various compositions (25/75, 50/50, 75/25). Films of copolymers and blends, as well as of a homopolymer sample (iPP), obtained by compression moulding and with different thermal history were characterized by optical and scanning electron microscopy (OM, SEM), small-angle light scattering (SALS), small- and wide angle X-ray scattering (SAXS, WAXS) and differential scanning calorimetry (DSC). It was found that all copolymers and blends studied crystallized exclusively in monoclinic α-modification forming spherulitic structure in a very broad undercooling range. The average size of spherulites is smaller in the copolymer containing 1-butene as compared to those containing ethylene or to iPP homopolymer, due to enhanced heterogeneous nucleation in BP copolymer. SEM microscopic observations demonstrated that EP and BP copolymers were miscible at all examined compositions and form homogeneous blends. Structural and morphological analysis indicated that the comonomer units are incorporated into growing crystallites in both EP and BP copolymers, while the non-crystallizing material is rejected out of the crystallites. For small concentrations of comonomer some of non-crystallizing species are pushed ahead of the front of growing spherulite into interspherulitic regions. For higher comonomer concentration these species are mostly trapped in intraspherulitic regions. Melting behavior of copolymers reflects the incorporation of comonomer into crystalline phase: melting temperature and crystallinity degree decrease in copolymers and blends as compared to plain iPP.     

Effects of shearing and comonomer content on the crystallization behavior of poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate)

Poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate) (PBSEMS) random copolymers were prepared with different comonomer compositions. The effects of shearing and comonomer content on the crystallization behavior of these copolymers were investigated at 80 °C. The thermal and morphological properties of the resulting samples were also discussed. The copolymers showed a longer induction time and a slower crystallization rate with increasing comonomer content. The promoting effect of shear on the overall crystallization behavior was more notable for those copolymers containing more 2‐ethyl‐2‐methyl succinic acid (EMSA) units. The melting temperature of ‘as‐prepared’ poly(butylene succinate) (PBS) was ca. 115 °C, while that of the copolymers varied from 112 to 102 °C. Higher comonomer contents in the copolymers gave rise to lower melting temperatures and broader melting peaks. In addition, the isothermally crystallized samples showed multiple melting endothermic behavior, the extent of which depended on the comonomer content. The copolymers showed different wide‐angle X‐ray diffraction (WAXD) patterns from that of neat PBS, depending on the comonomer content and shear applied during crystallization. With increasing comonomer content, the copolymers crystallized without shearing, showing the shifting of a diffraction peak to a higher angle, while those crystallized under shear did not show any peak shift. Copyright © 2004 Society of Chemical Industry     

The influence of thermo‐mechanical history on structure development of elastomeric and amorphous glass thermoplastic polyurethanes

In this work, the effect of processing and subsequent thermal treatment on the rheological behavior and microstructure aggregation of two different types of thermoplastic polyurethanes (TPUs) are investigated. Elastomeric TPUs, which are segmented block copolymers composed of alternating soft and hard segments, and amorphous glassy TPU, predominantly composed of hard segments, were subjected to heating/cooling cycles after extrusion. All materials show rheological hystereses after the first thermal cycle; in amorphous glassy TPUs, there is an increase in dynamic moduli but the loss tangent remains unchanged, whereas in the elastomeric TPUs, there are both decreases in dynamic moduli and hystereses in the loss tangent. The changes in molecular weight during the thermal cycle were tracked by gel permeation chromatography and shown not to be responsible for the rheological hystereses. For both elastomeric and amorphous glassy TPUs, fourier transform infrared spectroscopy results indicate that an increase in dynamic moduli, especially G′, is associated with a higher degree of hydrogen bonding. These results combined with X‐ray data indicate that upon extrusion the elastomeric TPUs form a network in which hydrogen bonding plays an important role. This structure can be disrupted upon subsequent heating and cooling. POLYM. ENG. SCI., 54:1383–1393, 2014. © 2013 Society of Plastics Engineers     

Interphase effects on the mechanical and physical aspects of natural fiber composites

The interaction and adhesion between the fiber and matrix has a significant effect in determining the mechanical and physical behavior of fiber composites. The effect of the interface and interphase depends on several factors such as chemical composition (functional groups), molecular structure characteristics (branching, molecular weight distribution, cross-linking), and details of its physical state (above or below T_g, nature and degree of crystallinity). Natural fibers have complex and varying chemical structures that have uneven surface topographies. This creates difficulties in using single fiber composite testing to accurately evaluate the interfacial shear strengths, except for comparisons. A review of our interphase related research in natural fiber composites is presented. When using coupling agents it is well known that the tensile and flexural strengths increase dramatically in natural fiber reinforced composites. However, in the case of modulus, the results are more complex. For two ethylene-propylene impact copolymers, the uncoupled systems had much higher Young's moduli than the coupled systems. The dynamic storage moduli of the uncoupled impact polymers were higher than the coupled composites at temperatures up to about 50°C. At higher temperatures the presence of the coupling agent resulted in higher storage moduli. Transcrystallinity may play an important role in this phenomenon. Creep and other long-term properties are also affected by the quality of the interphase, although the level of improvement decreases with an increase in the molecular weight of the matrix polymer. Coupling agents reduced the rate of water absorption and the moduli were less affected in blends with a higher concentration of coupling agents.     

Effect of polymerization conditions on the melt rheological properties of vinyl chloride–vinyl acetate copolymers

Mathematical models have been developed which predict the composition, molecular weight, and melt rheological properties for vinyl chloride/vinyl acetate copolymers of inherent viscosity range 0.4–0.7 dL/g and bound vinyl acetate levels of 3.8–17.4%. The effect of polymer long chain branching on the viscous/elastic moduli ratio is discussed as well as the comparison of Tinius–Olsen melt index measurements vs. mechanical spectrometer results. The reactivity ratio for vinyl chloride/vinyl acetate comonomer pairs was remeasured and found to be significantly different from literature values.     

Copolymerization of ethylene and 1-octadecene with the Cp2ZrCl2/MAO and Cp2HfCl2/MAO catalyst systems

Summary The comparison of the copolymers obtained with the Cp₂ZrCl₂/MAO and Cp₂HfCl₂/MAO catalyst systems showed that the catalyst having hafnocene was much more reactive towards 1-octadecene than zirconocene. The comonomer concentration had to be three times higher in the zirconocene copolymerization than in the hafnocene copolymerization when the level of 6 mol-% was reached. Although the hafnocene catalyst was more reactive towards 1-octadecene, the molecular weights were higher than in the copolymers obtained with the zirconocene catalyst.The total activity of the zirconocene was 10 times higher than with the hafnocene catalyst. With the zirconocene catalyst the activity towards ethylene was constantly increasing by increasing the comonomer concentration but stayed nearly constant with the hafnocene catalyst. It seemed that there is no rate enhancement effect upon comonomer addition with the hafnocene catalyst.     

Elongational viscosities of random and block copolymer melts

The influence of random and block copolymerized structures on the uniaxial elongational viscosity was investigated. The investigated random copolymers were poly(ethylene-random-ethyl methacrylate) with comb-branched structure and poly(styrene-random-acrylonitrile) with linear structure. The studied block copolymers were poly(styrene-block-ethylenebutylene-block-styrene) with linear structure. The elongational viscosities of random copolymers showed strain-hardening properties. The strain-hardening property was influenced little by comonomer contents and depended on whether copolymers had linear or branched structures. In contrast, the elongational viscosities of block copolymers gave strain-softening properties. The strain-softening property was not affected by strain rates and block comonomer ratios. The causes of strain-hardening and -softening properties are discussed from relaxation spectrum and damping function based on the Bernstein–Kearsley–Zapas model. The damping functions of linear and branched random copolymers agreed with those of linear and branched homopolymers, respectively. On the other hand, linear block copolymers exhibited stronger damping than linear homopolymers. It was concluded that strain-hardening and -softening properties in the elongational viscosity of random and block copolymerized structures are correlated with their damping functions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 1765–1774, 1998     

Terpolymers. I. The mechanical properties and transition temperatures of terpolymers of n-octadecyl acrylate,ethyl acrylate,and acrylonitrile

Earlier work revealed that the internal plasticization of polyacrylonitrile by the higher n-alkyl acrylates or N-n-alkylacrylamides yielded only brittle copolymers. This difficulty was circumvented in the present work by starting with copolymers of acrylonitrile and ethyl acrylate, over the range of compositions, and further modifying these by incrementally displacing the ethyl acrylate in each recipe by n-octadecyl acrylate through terpolymerization. In this way, the stepwise small reduction in T_g for the base ethyl acrylate–acrylonitrile copolymers was greatly increased for each of the terpolymers. Compositions were obtained ranging from glassy, brittle terpolymers, with glass transitions above room temperature, to soft plasticized polymers having sufficient polar networks retained from the nitrile to confer useful properties. The decline in the glass temperature was shown to be dependent on the free volume conferred by the side-chain methylene groups of each acrylate ester. In contrast, the decline in tensile and flexural strengths and moduli for the terpolymers having glass transitions above room temperature was produced entirely by the presence of the methylene groups of the 18-carbon ester. The glass transition region corresponded to room temperature when the acrylonitrile content of the base copolymer had been reduced to 50 mole-%. Terpolymers of this nitrile content and lower had the low moduli and large elongations of plasticized compositions. An equation was developed which correlated empirically the glass transitions and the mechanical properties with the weight fraction of the acrylate esters for the glassy terpolymers.     

Effect of molecular structure and rheology on the compression foam molding of ethylene‐α‐olefin copolymers

The morphology and mechanical properties of foams made out of a series of ethylene‐α‐olefin copolymers having well‐characterized rheological properties were investigated. A compression foaming molding technique was implemented, using azodicarbonamide as the blowing agent. The polymers differed in the amount of comonomer contained (resulting in a range of densities), type of comonomer (octene vs. butene) and molecular weight, resulting in variable thermal properties and different rheological responses under shear and extensional flow. The results showed that the majority of the octene‐based copolymers with comparable rheological properties had similar foam morphology. A distinct behaviour was observed for the butene‐based copolymer, as well as the octene‐containing one having the lowest density and lowest melting/crystallization points. The poor foamability of these grades was attributed to their differences in extensional and thermal properties, respectively. Increasing density resulted in a higher secant modulus of the foamed samples. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Terpolymers. III. Isochronal viscoelastic parameters for terpolymers of vinyl stearate,vinyl acetate,and vinyl chloride

Isochronal viscoelastic parameters were collected for many of the copolymers, terpolymers, and diluent mixtures whose mechanical properties at ambient temperatures were reported in the preceding paper. In the polymeric systems, vinyl stearate, acting as the primary internal plasticizer, was introduced into terpolymers by displacing vinyl acetate from base copolymers of vinyl acetate and vinyl chloride, across the range of composition. In the diluent mixtures, poly(vinyl chloride) was plasticized by di-2-ethylhexyl phthalate across the range of compositions. For direct comparison with the mixtures, vinyl chloride was plasticized by copolymerization with vinyl stearate across the same range of compositions. Moduli for the co- and terpolymers reached the low values characteristic of soft materials at room temperature only through a short range of vinyl stearate composition. At higher internal plasticizer compositions, side-chain crystallization stiffened the samples and raised their moduli. In contrast, moduli for the mixtures decreased steadily with increase in diluent at ambient temperature. The effective use temperature ranges were narrow for the co- and terpolymers but broad for the mixtures. Curve broadening was similar for both types of systems, but reached a maximum at about 40 weight-% plasticizer for the diluent mixtures. The slopes of the glassy modulus with decreasing temperature at 50°C below T_g for the vinyl stearate copolymers were relatively large. However, moduli close to that of poly(vinyl chloride) were reached only near the temperature range associated with the γ-transition. Consequently, this behavior was attributed to motions of the side chains in the glassy matrix. Room temperature moduli, which could be obtained before the onset of melting, were correlated with the fractional side-chain crystallinity for polymers having a high vinyl stearate content. From this relation, the modulus for the hexagonal crystal form of the side-chain crystallites of poly(vinyl stearate) was estimated to be 1.2×10¹⁰ dynes/cm². Moduli for the glassy amorphous phase of this same polymer appeared to have one sixth of this value at 40°C below the glass transition. The glass transition temperature occurred about 10° below the inflection temperature at 10⁹ dynes/cm², as an average for all of the systems studied.     

Melting behavior of polyacrylonitrile copolymers

Summary The melting behavior of acrylonitrile copolymers, ter- and tetrapolymers was studied in the dry state and in the presence of water. The melting point depression caused by the incorporation of a specific comonomer into the polyacrylonitrile chain was shown to be dependent on the molecular structure of the comonomer. Not all comonomers gave equivalent melting point depressions on a molar basis. The Eby theory of comonomer melting was used to model the melting behavior. This theory assumes that the non-crystallizing (non-AN) comonomers enter the crystal lattice as point defects rather than being relegated to the amorphous phase. An equation was developed for predicting the melting point of copolymers, terpolymers and higher order polymers as a function of the polymer composition and the specific melting point depression constant for each comonomer. The latter constants are derived from the copolymer melting point curves. The equation is applicable to both dry and wet polymers and excellent agreement between the observed and calculated melting points for wet terpolymers and tetrapolymers was observed.     

Dynamic mechanical behaviour of random copolymers of a LC-methacrylate and octyl methacrylate

The dynamic mechanical behaviour of random copolymers of LC monomer-1-(hexyloxycarbonyl)ethyl 4-[4-(methacryloyloxy)benzoyloxy]benzoate (HB) and octyl methacrylate (OMA) was studied in the main transition and flow regions. Even though the aliphatic end groups of the side chain of HB and OMA are roughly the same, the T _g temperature of poly(HB) is ∼ 80 K higher than that of poly(OMA); this fact is due to the presence of the stiff phenyl benzoate mesogenic group in the side chain of HB. With increasing content of OMA in the copolymer the superimposed curves of the storage G′ _p and loss G′′ _p moduli at a constant temperature shift towards shorter frequencies. It has been shown that this shift is mainly due to an increase of the free volume in the copolymers with increasing content of OMA. While HB monomer shows liquid crystalline (LC) properties, its polymer (poly(HB)) and random copolymers with OMA show only isotropic thermal behaviour because no flexible spacer is present in the side chain of HB which would decouple the main chain and mesogenic group motions. This means that neither the homopolymer of HB, nor its copolymers with a flexible comonomer retain the LC properties of the starting LC monomer, HB. Received: 26 September 1996/Revised: 7 November 1996/Accepted: 7 November 1996     

Isothermal crystallization of metallocene‐based propylene/α‐olefin copolymers

Isothermal crystallization and subsequent melting behavior of two propylene/hexene‐1 copolymers and two propylene/octene‐1 copolymers prepared with metallocene catalyst were investigated. It is found that γ‐modification is predominant in all copolymers. The Avrami exponent shows a weak dependency on comonomer content and comonomer type. At higher crystallization temperatures (T_c) the crystallization rate constant changes more rapidly with T_c and the crystallization half‐time substantially increases. Double melting peaks were also observed at high T_c, which is attributed to the inhomogeneous distribution of comonomer units along the polymer chains and the existence of crystals with different lamellar thicknesses. The equilibrium melting temperatures (T) of the copolymers were obtained by Hoffman–Weeks extrapolation. It was found that the T decreases with increasing comonomer content, but are independent of comonomer type, implying that comonomer units are excluded from the crystal lattice. Dilation of the crystal lattice was also observed, which depends on crystallization, comonomer content, and comonomer type. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 240–247, 2005     

Thermal fractionation and effect of comonomer distribution on the crystal structure of ethylene-propylene copolymers

In this paper the comonomer distributions of two series of ethylene-propylene copolymers with different propylene contents, which were prepared by a fluorinated bis(phenoxyimine) Ti catalyst (FI-EP copolymers) and a conventional Ziegler-Natta catalyst (ZN-EP copolymers), respectively, were characterized. It is found that the comonomer distribution of ethylene-propylene copolymers can still be characterized by thermal fractionation at a long scale, though the propylene units can be incorporated into the PE crystal lattice. The FI-EP copolymers exhibit a narrow and random comonomer distribution, whereas a broad comonomer distribution is observed for the ZN-EP copolymers. The crystal structures of the FI-EP and ZN-EP copolymers were studied by WAXD. The a-axis of the PE crystals of the FI-EP copolymers increases rapidly with propylene content, indicating that more propylene units are incorporated into the PE crystal lattice, whereas only a slight expansion in a-axis is observed for the ZN-EP copolymers. WAXD result also shows the presence of hexagonal phase in the FI-EP copolymers and the relative content of the hexagonal phase increases with the propylene content, while in the ZN-EP copolymers the hexagonal phase is negligible.     

Study of the effect of the monomer pressure on the copolymerization of ethylene with 1-hexene

The effect of ethylene pressure on the copolymerization of ethylene with 1-hexene was studied. The results show an increasing of productivities (g of polymer/n_Zr h) with pressure. This tendency was not observed for the activity (g of polymer/n_Zr h bar) that decreases when pressure is raised. When varing the pressure, the characteristics and properties of the formed copolymers are in accordance with the expectation for changes in the monomer concentration; increasing the pressure causes a decrease in comonomer incorporation. At higher ethylene pressure, the polymer is more crystalline due to less incorporation of 1-hexene and the molecular weight is higher. The density of the copolymers also decreases with comonomer incorporation into the copolymer © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2567–2574, 1997     

Radiation-initiated graft copolymerization of individual monomer and comonomer onto polyethylene and polytetrafluoroethylene films

Preparation and some properties of the graft copolymers obtained by radiation-induced graft polymerization of methacrylic acid (MAA) and (acrylonitrile/MAA) comonomer onto polyethylene and polytetrafluoroethylene films, were investigated. The effect of reaction conditions, solvent, monomer, and inhibitor concentration, comonomer composition, and comonomer concentration, on the graft copolymerization process was studied. The grafting process was enhanced in the presence of comonomer (AN/MAA) as compared with individual grafting of MAA or acrylonitrile (AN). The optimum comonomer composition, at which the highest grafting yield was obtained, was found to be (80/20) wt % of (AN/MAA) comonomer. The graft copolymerization of (AN/MAA) comonomer was enhanced in presence of AN due to its higher polarity strength. The electrical and swelling properties of the graft copolymers were greatly affected by the contents of PAN and PMAA graft chains. Mechanical properties of the graft copolymers were significantly changed with the grafting yield.     

Rheological,thermal and crystallisation properties of ethylene,propylene and α-olefin copolymers. I Rheological properties

Abstract

A series of random ethylene, propylene/1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene copolymers, ethylene and propylene homopolymers were prepared and investigated. The rheological properties (steady state and dynamic shear viscosity, creep compliance and plateau modulus), of copolymer samples with different co-unit content and molecular masses were determined and compared with the properties of homopolymers. The effects of the length of counit and the comonomer content were investigated. The copolymers exhibited similar rheological properties to the homopolymer but they have a lower shear viscosity, normal viscosity, higher steady state creep compliance and smaller plateau modulus values. The effect of comonomer content was evaluated on the bases of free volume theory.     

Copolymers of ethylene with butene-1 and long chain α-olefins. I. Decene-1 as long chain α-olefin

The possibility was studied of using decene-1 as comonomer with ethylene in a slurry type polymerization with Ziegler–Natta catalysts. Under the reaction conditions used decene contents remained at the 2 wt% level in ethylene/decene-1 copolymers. When additionally butene-1 was present in the polymerization, decene-1 contents were significantly higher. A synergistic effect was identified in the reactivities of butene-1 and decene-1 in terpolymerization with ethylene. The comonomer reactions were determined and comonomer contents measured by ¹³C-NMR spectroscopy. Decene-1 content had an effect on the polymer density and crystallinity, but virtually no effect on melting temperature. With high comonomer contents an additional melting range was identified in DSC curves at about 100°C.