M. van Gurp

Hydrogen‐bonded supramolecular polymer networks

The strong dimerizing, quadruple hydrogen-bonding ureido-pyrimidone unit is used to obtain reversible polymer networks. A new synthetic route from commercially available starting materials is described. The hydrogen-bonding ureido-pyrimidone network is prepared using 3(4)-isocyanatomethyl-1-methylcyclohexyl-isocyanate (IMCI) in the regioselective coupling reaction of multi-hydroxy functionalized polymers with isocytosines. 1H- and 13C-NMR, IR, MS, and ES-MS analysis, performed on a model reaction using butanol, demonstrated the formation of the hydrogen-bonding ureido-pyrimidone unit in a yield of more than 95%. The well-defined, strong hydrogen-bonding ureido-pyrimidone network is compared with a traditional covalently bonded polymer network, a multi-directional hydrogen-bonded polymer network based on urea units, and a reference compound. The advantage of the reversible, hydrogen-bonded polymer networks is the formation of the thermodynamically most favorable products, which show a higher “virtual” molecular weight and shear modulus, compared to the irreversible, covalently bonded polymer network. The properties of the ureido-pyrimidone network are unique; the well-defined and strong dimerization of the ureido-pyrimidone unit does not require any additional stabilization such as crystallization or other kinds of phase separation, and displays a well-defined viscoelastic transition. The ureido-pyrimidone network represents the first example of a truly reversible polymer network showing these features. Furthermore, the ureido-pyrimidone dimerization is strong enough to construct supramolecular materials possessing acceptable mechanical properties.

Competitive reactions during compatibilization of blends of polybutyleneterephthalate with epoxide-containing rubber

Blends of polybutyleneterephthalate (PBT) with ethene-ethyl acrylate copolymer (E-EA) and ethene-methyl acrylate-glycidyl methacrylate terpolymer (E-MA-GMA) were investigated. E-MA-GMA terpolymers containing various concentrations of epoxide functions were synthesized in a preliminary step by melt modification of commercial E-MA-GMA with benzoic acid molecules. The blends were analyzed by several techniques including electron microscopy and separation experiments. PBT/E-EA (80/20 w/w) blends presented the general features of uncompatibilized polymer blends, such as a lack of interfacial adhesion and a relatively coarse unstabilized morphology. No evidence of transesterification reaction was found to occur according to the used blending conditions. In contrast, blends containing both virgin and modified E-MA-GMA terpolymers exhibited a very complex behavior. Fractionation experiments demonstrated that two competitive reactions take place during the melt blending viz. (1) compatibilization due to interfacial reactions between PET chains end and terpolymer epoxide groups, resulting in the formation of E-MA-GMA/PBT graft copolymer and (2) rapid crosslinking of the rubber phase due to the simultaneous presence of hydroxyl and epoxide groups on E-MA-CMA chains. This competition between compatibilization and crosslinking is clearly dependent on the type of the terpolymer, since the modified E-MA-GMA already contains hydroxyl groups before mixing but for the pure E-MA-GMA hydroxyl groups are formed as a result of the in situ compatibilization reaction with PET. Rubber phase crosslinking through double reaction with PET chain can not be excluded but is expected to occur only to a smaller extent. All these phenomena result in a very complex processing/morphology interrelationships and probably also affects the final blend properties

The use of rotation matrices in the mathematical description of molecular orientations in polymers

The orientational distribution of chain segments or crystals in polymeric materials is most conveniently described as a series expansion of rotation matrix elements. Rotation matrices are representations of rotation operators, and their elements form a complete set of orthogonal functions. Their use in the mathematical description of molecular orientations therefore yields the advantage that the moments in the series expansion of the orientational distribution, also called order parameters, are the averages of the rotation matrix elements themselves. Furthermore, rotations between molecular and macroscopic frames can easily be carried out to link molecular orientation to a macroscopic observable. In the third place, various symmetries reduce the number of non-vanishing order parameters in a straightforward way. In the case of fiber symmetry and axially symmetric molecular elements, the rotation matrix elements reduce to Legendre polynomials.The mathematical formalism can be applied to any of the various techniques used to measure the degree of orientation in polymers. IR-dichroism and birefringence yield order parameters of rank 2 only. Fluorescence depolarization and polarized Raman scattering yield order parameters of rank 2 and X-ray scattering and NMR may yield order parameters of higher rank, dependent on the symmetry of the system and the experimental accuracy.From experimentally derived order parameters, the most probable orientational distribution function can be constructed, using the maximum entropy formalism.

Reactive compatibilization of blends of polybutyleneterephthalate with epoxide-containing rubber. The effect of the concentrations in reactive functions

The influence of the functional groups concentrations on the reactive compatibilization of polybutylene terephthalate (PBT)/epoxide-containing rubber blends has been investigated by using various PBT and ethene-(methyl acrylate)-(glycidyl methacrylate) terpolymer (E-MA-GMA) grades. The reactivity of the rubber phase was modified using different strategies, including the use of commercial terpolymer grades, modification of E-MA-GMA or diluting E-MA-GMA terpolymers with non-reactive ethene-(methyl acrylate) (E-MA) copolymer. The reactive blends were analyzed amongst others by electron microscopy and fractionation experiments. It was shown that the final particle size is directly related to the amount of copolymer formed in situ at the blend interface, i.e. chain area density Sigma. A value of approximately 0.05 chains/nm(2) for Sigma is necessary to suppress dynamic coalescence and to obtain very fine dispersion (<0.2 mum). This requires a sufficiently high concentration in reactive functions at the interface vicinity. In this context, the time required for equilibrium morphology is rather independent of the GMA content in the terpolymer but is, however, intimately related to the PBT chain ends concentration. Investigation of PBT/(E-MA-GMA/E-MA) ternary blends revealed unambiguously that the formation of the copolymer at the interface is not controlled by the diffusion of the reactive chains towards the interface. The performed experiments offer new opportunities for modulating the final morphology and the properties of the PBT/rubber blends

On the Development of Orientation in LDPE Blown Films

The orientational order in a number of low density polyethylene (LDPE) films blown at varying extrusion temperatures is studied with four dif ferent experimental techniques: wide-angle X-ray scattering (WAXS), Fourier- transform infrared (FTIR) dichroism, polarized Raman spectroscopy and bire fringence. From these experimental studies a number of so-called order parameters is obtained that make up the orientational distribution function of the lamellar crystals in the films. The findings support the morphological model that consists of arrays of stacked lamellae which are twisted. These lamellae grow radially outward from a nucleation center along the crystalline b-axis, whereas the a- and c-axes rotate around this direction. With increasing extrusion temperature an increasing amount of twisting takes place, due to a reduced melt stress at the time of crystallization and a decreased number of aligned nuclei being formed. Furthermore, with increasing extrusion tempera ture the small amount of amorphous order decreases to practically zero above an extrusion temperature of around 150°C. From a comparison of the four experimental techniques it can be concluded that WAXS is the most comprehensive choice, yielding pole figures which directly show a qualitative picture of the crystalline orientational distribution. FTIR and Raman give a limited number of order parameters, but are less elabo rate in use than WAXS. Birefringence measurements are easily performed, but only give average values of both the crystalline and amorphous order. Using crystalline values from other measurements, these birefringence measure ments can then be used to obtain the amorphous order.

Odd-rank order parameters in non-uniaxial orientational distributions

Abstract The consequences of double-reflection symmetry operations for the orientational distribution function are discussed. A double-reflection operation is equivalent to a rotational transformation, and conserves the handedness of the molecular and macroscopic frames that are transformed. It is found that, in general, non-vanishing order parameters of both even and odd order exist for a biaxial phase containing biaxial molecules. This finding is confirmed by numerical computations of the order parameters using a simple form for the orientational distribution function.

Rheological characterization of low-density polyethylene in planar extension using rheo-optics

Optical techniques such as birefringence and Laser Doppler velocimetry can be used to characterize the flow behavior of polymeric liquids in complex flow geometries. This use of optical techniques is called rheo-optics. In the study reported here, rheo-optical measurements were carried out on a low density polyethylene (LDPE) melt in a converging rectangular slit die. Using Laser Doppler velocity measurements as input data, extensional stresses along the center line of the converging slit were calculated using two different constitutive equations, the Wagner equation and the Phan-Thien and Tanner (PTT) equation. The calculated curves were compared with experimental stress data as obtained from birefringence experiments. It was found that calculations based on the PTT model give a better agreement with the experimental data than Wagners equation. The application of rheo-optical measurements has the great advantage that local information is obtained on both the stress field and the velocity field in non-homogeneous flows. However, a number of possible causes of errors are introduced that should carefully be considered. Some of the problems involved in rheo-optical experiments are discussed.

Molecular Orientation and Mechanical Properties of Poly(Vinyl Alcohol) Fibers

Abstract A number of poly(vinyl alcohol) fibers with different draw ratios was characterized by measuring the birefringence, crystalline orientational order, crystallinity, tensile strength, and modulus. The birefringence, tensile strength and modulus increased with increasing draw ratio whereas the crystallinity and crystalline order parameters remained constant within narrow limits. The increase in birefringence has to be attributed solely to an increase in chain orientation in the amorphous phase of the semicrystalline fiber. The tensile strength and modulus are therefore directly related to the chain orientation in the amorphous phase. With the aid of a simple two-phase model it was found that the modulus of the amorphous phase in its disordered conformation was 4.8 GPa. The intrinsic birefringence of the amorphous phase was found to be 79 × 10−3, i.e. much higher than the value obtained for the crystalline phase (52 × 10−3). When this value was used in calculations, it was found that the order parame...

A Method for Studying Pigment Organization in Photosynthetic Complexes

The use of polarized light spectroscopy is an important way to study pigment-organization in photosynthetic particles. For instance, linear dichroism (LD) measurements are often used to obtain information about the orientations of absorption transition moment vectors within these particles, which is interesting in view of understanding excitation energy transfer, but it may also lead to knowledge about the pigment organization. To perform LD-measurements, particles are often ordered in biaxially compressed polyacrylamide gels. Knowledge about the average orientation of the particles in the gel is required, to obtain quantitative information about the average transition moment directions. The orientation distribution is often badly known. Mathemetical expressions for rod-like and disc-like particles have been presented by Ganago et al. [1], which relate the amount of gel-compression to the average particle orientation. Recently, we showed that for rod-like chlorosomes of Chloroflexus aurantiacus, ordered in gels, the formalism described in [1] does indeed lead to a good estimation of the average orientation [2].Unfortunately, mathematical expressions are not available for particles, which are not rod- or disc-like.