Tadeusz Pakula

Gradient copolymers by atom transfer radical copolymerization

Gradient copolymers, in which the instantaneous composition varies continuously along each chain, are discussed in terms of theoretical background, significance and examples from the literature. The specific focus is the use of atom transfer radical polymerization to synthesize gradient copolymers with various composition profiles. Examples of gradient copolymers using ionic and controlled radical polymerization techniques are presented. Atom transfer radical polymerization has been used under both batch and semi-batch conditions to produce gradient copolymers. The physical properties of gradient copolymers and possible future work in the field of gradient copolymers are discussed. Copyright

Simple and effective one‐pot synthesis of (meth)acrylic block copolymers through atom transfer radical polymerization

The synthesis of di- and triblock copolymers using atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) and methyl methacrylate (MMA) is reported. In particular, synthetic procedures that allow for an easy and convenient synthesis of such block copolymers were developed by using CuBr and CuCl salts complexed with linear amines. Polymerizations were successfully conducted where the monomers were added to the reactor in a sequential manner. Poor cross-propagation between poly(n-butyl acrylate) (PBA) macroinitiators and MMA was minimized, and therefore control of molecular weights and distributions was realized, by using halogen exchange—a technique involving the addition of CuCl to the MMA during the chain extension of the PBA macroinitiator. High molecular weight (Mn ∼ 90,000) and low polydispersity (Mw /Mn < 1.35) ABA triblock copolymers were also prepared and their structure and properties in bulk have been preliminary characterized indicating the potential of ATRP for the production of all-acrylic thermoplastic elastomers.

On the shape of bottle-brush macromolecules: Systematic variation of architectural parameters

We measured the form factor of bottle-brush macromolecules under good solvent conditions with small-angle neutron scattering and static light scattering. The systems under investigation are brushes, synthesized via the grafting-from route, built from a poly(alkyl methacrylate) backbone to which poly(n-butyl acrylate) side chains are densely grafted. The aim of our work is to study how the systematic variation of structural parameters such as the side chain length and backbone length change the conformation of the polymer brushes in solution. All spectra can be consistently described by a model, considering the bottle-brush polymers as flexible rods with internal density fluctuations. Parameters discussed are (1) the contour length per main chain monomer l(b), (2) the fractal dimension of the side chains Ds, as well as (3) the fractal dimension D, and (4) the Kuhn length lambdak of the overall brush. l(b)=0.253+/-0.008 nm is found to be independent of the side chain length and equal to the value found for the bare main chain, indicating a strongly stretched conformation for the backbone due to the presence of the side chains. The fractal dimension of the side chains is determined to be Ds=1.75+/-0.07 which is very close to the value of 10.588 approximately 1.70 expected for a three-dimensional self-avoiding random walk (3D-SAW) under good solvent conditions. On larger length scales the overall brush appears to be a 3D-SAW itself (D=1.64+/-0.08) with a Kuhn-step length of lambdak=70+/-4 nm. The value is independent of the side chain length and 46 times larger than the Kuhn length of the bare backbone (lambdak=1.8+/-0.2 nm). The ratio of Kuhn length to brush diameter lambda(k)d>or=20 determines whether lyotropic behavior can be expected or not. Since longer side chains do not lead to more persistent structures, lambda(k)d decreases from 8 to 4 with increasing side chain length and lyotropic behavior becomes unlikely.

Monte Carlo simulation of lattice models for macromolecules at high densities

Polymer chain models on quadratic, cubic, triangular, and face‐centered cubic lattices at volume fractions near unity are simulated with a Monte Carlo algorithm which transports beads from kinks or chain ends along the chain contour to another position of the chain by a slithering motion of the intervening chain part. Special cases are the slithering snake motion where the whole chain takes part in the slithering motion and a conformation change of a kink or an end group. For dense systems it is found that this algorithm is much more efficient than the slithering snake algorithm or algorithms which use only local motions. It can be used with good efficiency even for systems at a volume fraction of unity by moving chain parts collectively (collective motion algorithm). The computed chain dimensions agree with data obtained from other algorithms and with literature data.

Computer simulation of polymers in thin layers. I, Polymer melt between neutral walls - static properties

Systems of linear chains on the face centered cubic lattice with all lattice sites occupied are simulated by the cooperative motion method. Static properties of chains are analyzed and various statistical models of chain shapes are presented, differing in details of individual chain states taken into account. The structure of thin layers of a polymer melt confined between two parallel neutral walls is compared with the melt structure in bulk. It is shown that the effect of neutral walls consists mainly in reorientation of cigar‐shaped chains in the vicinity of walls, but not in changes of dimensions and shapes of chains.

Aromatic polyamides with pendent acetoxybenzamide groups and thin films made therefrom

Abstract New aromatic polyamides containing 1,3,4-oxadiazole or benzonitrile units in the main chain and 5-(4-acetoxybenzamido) groups in the side chain have been synthesized and their properties have been characterized and compared with those of related polyamides and polyoxadiazole-amides. These polymers show good thermal stability, with initial decomposition temperature being at about 300 °C and glass transition temperature in the range of 260–280 °C. They are easily soluble in certain solvents such as N-methylpyrrolidinone (NMP), N,N′-dimethylacetamide (DMA) and N,N′-dimethylformamide (DMF) and can be cast from solutions into thin flexible films. The polymer films had tensile strengths in the range of 77–97 MPa, tensile moduli in the range of 2.3–2.6 GPa and elongation at break values ranging from 6% to 24%. One of the polymers containing the 1,3,4-oxadiazole ring exhibited blue fluorescence.

Solid-state morphologies of linear and bottlebrush-shaped polystyrene-poly(Z-L-lysine) block copolymers

Abstract The solid-state structures of polystyrene–poly(Z- l -lysine) block copolymers were examined with respect to the polymer architecture and the secondary structure of the polypeptide using circular dichroism, quantitative small- and wide-angle X-ray scattering, and electron microscopy. Linear block copolymers exhibit a hexagonal-in-lamellar structure where folded and packed polypeptide α-helices form lamellae which extend over an exceptional broad range of the composition diagram. Star- or bottlebrush-shaped copolymers are able to stabilize a larger interface area than linear ones which promotes the formation of undulated lamellar mesophases. Depending on the secondary structure of polypeptide segments, plane lamellar, superundulated lamellar, or corrugated lamellar phases are formed. These results indicate the importance of a secondary structure and packing of polymer chains for the formation of new phases and ordering far from the ‘classical’ phase behavior.

Redox-active liquid-crystalline ionomers: 1. Synthesis and rheology

Abstract Liquid-crystalline (LC) copolymers with redox-active groups were prepared by copolymerization of mesogenic and ferrocene-containing monomers (up to 10%). In these copolymers the ferrocene groups can be oxidized reversibly to prepare ionomers, while retaining the LC phase. It is thus possible to vary the amount of ionic groups in the ionomers by a redox reaction. The oxidized (charged) polymers show a strong excess X-ray scattering at small angles. This is typical for ionomers and is assigned to the scattering of ionic aggregates. Dynamic mechanical measurements show that these aggregates are effective as crosslinking points. Thus an oxidation-reduction reaction can be used to transfer an uncrosslinked polymer (reduced ferrocene groups, uncharged) reversibly into an oxidized polymer that acts like a weakly crosslinked gel.

Relaxation and viscoelastic properties of complex polymer systems

Dynamic behavior of various polymer melts is studied on the basis of a comparison of viscoelastic properties with the information obtained from dielectric spectroscopy. The experimental observations are compared with results of computer simulation of corresponding systems. The studies include simple melts of linear chains, block copolymer systems of miscible components, as well as the behavior of melts with molecular objects of complex topology-like stars or microgels. In the case of polyisoprene linear chain melts an equivalence of terminal relaxation times determined from mechanical and dielectric measurements is demonstrated. Using linear block copolymers of isoprene and butadiene, relaxation times of chain fragments (isoprene blocks) in relation to relaxation times of whole copolymer chains are determined and compared with theory and simulation. Both the experimentally determined block relaxation times and relaxation times of chain fragments in simulated linear chain melts show a disagreement with predictions of the reptation theory. In the case of multiarm star polymers and microgel melts, the slow relaxation modes observed in viscoelastic spectra are assigned to cooperative translational motions detected in corresponding simulated systems in which an ordering of such molecules is demonstrated. This suggests that the terminal relaxation in multiarm star or microgel melts is governed by another relaxation mechanism than in linear chain melts. High efficiency of the Cooperative Motion Algorithm in simulation of dense systems of complex molecules is demonstrated.

Preparation of polyisobutene-graft-poly(methyl methacrylate) and polyisobutene-graft-polystyrene with different compositions and side chain architectures through atom transfer radical polymerization (ATRP)

Polyisobutene-graft-poly(methyl methacrylate) and polyisobutene-graft-polystyrene with controlled compositions and side chain architectures were prepared through atom transfer radical polymerization (ATRP). Poly[isobutene-co-(p-methylstyrene)-co-(p-bromomethylstyrene)] (PIB) was used as a macroinitiator in the presence of CuCl or CuBr as a catalyst and dNbpy as a ligand. The compositions were controlled by the conversion of the monomer with polymerization time. The molecular weight distributions of the side chains were controlled through ATRP in the presence/absence of a halogen exchange reaction. DSC and DMA measurements showed that graft copolymers have two glass transition temperatures suggesting microphase separated behavior, which was also confirmed by SAXS measurements. The phase and dynamic mechanical behaviors were strongly affected by the compositions and/or the side chain architectures. The properties of the graft copolymers were controlled in a wide range leading to toughened glassy polymers or elastomers.