Giovanni Polacco

A review of the fundamentals of polymer-modified asphalts: Asphalt/polymer interactions and principles of compatibility

During the last decades, the number of vehicles per citizen as well as the traffic speed and load has dramatically increased. This sudden and somehow unplanned overloading has strongly shortened the life of pavements and increased its cost of maintenance and risks to users. In order to limit the deterioration of road networks, it is necessary to improve the quality and performance of pavements, which was achieved through the addition of a polymer to the bituminous binder. Since their introduction, polymer-modified asphalts have gained in importance during the second half of the twentieth century, and they now play a fundamental role in the field of road paving. With high-temperature and high-shear mixing with asphalt, the polymer incorporates asphalt molecules, thereby forming a swallowed network that involves the entire binder and results in a significant improvement of the viscoelastic properties in comparison with those of the unmodified binder. Such a process encounters the well-known difficulties related to the poor solubility of polymers, which limits the number of macromolecules able to not only form such a structure but also maintain it during high-temperature storage in static conditions, which may be necessary before laying the binder. Therefore, polymer-modified asphalts have been the subject of numerous studies aimed to understand and optimize their structure and storage stability, which gradually attracted polymer scientists into this field that was initially explored by civil engineers. The analytical techniques of polymer science have been applied to polymer-modified asphalts, which resulted in a good understanding of their internal structure. Nevertheless, the complexity and variability of asphalt composition rendered it nearly impossible to generalize the results and univocally predict the properties of a given polymer/asphalt pair. The aim of this paper is to review these aspects of polymer-modified asphalts. Together with a brief description of the specification and techniques proposed to quantify the storage stability, state-of-the-art knowledge about the internal structure and morphology of polymer-modified asphalts is presented. Moreover, the chemical, physical, and processing solutions suggested in the scientific and patent literature to improve storage stability are extensively discussed, with particular attention to an emerging class of asphalt binders in which the technologies of polymer-modified asphalts and polymer nanocomposites are combined. These polymer-modified asphalt nanocomposites have been introduced less than ten years ago and still do not meet the requirements of industrial practice, but they may constitute a solution for both the performance and storage requirements.

Bioartificial materials based on blends of collagen and poly(acrylic acid)

The interactions between soluble collagen (C) from calf skin and poly(acrylic acid) (PAA) were studied. Mixing aqueous solutions of collagen and PAA, at various pH values (2.5-4), leads to the formation of complexes that precipitate in the form of insoluble aggregates. The effects of mixture composition, pH, and ionic strength on C/PAA complex formation were investigated by gravimetric, turbidimetric, and conductometric analysis. The experimental results indicate that the complexes form through electrostatic interactions. Homogeneous solid films with variable C/PAA ratios were obtained by casting from solutions in which the pH was adjusted just over the isoelectric point of collagen, thus avoiding the attractive ionic interactions responsible for the complexation of collagen and PAA molecules. A relevant result obtained is related to the possibility of restoring the ionic interactions between the two polymers inside the solid films. Mixture composition and pH appear to influence the thermal properties of both complexes and films.

Reactive Compatibilizer Precursors for LDPE/PA6 Blends, 1 Ethylene/Acrylic Acid Copolymers

Ethylene/acrylic acid copolymers (EAA) with different acrylic acid (AA) contents have been used as compatibilizer precursors (CPs) for blends of two grades of lowdensity polyethylene (LDPE) with polyamide-6 (PA). In the first part of the work, binary blends of the CPs with LDPE and with PA have been studied in order to get an insight into the interactions of the EAA copolymers with the blends components. It has been shown that the CPs form immiscible, yet highly compatible, blends with LDPE. Investigation of the binary CP/PA blends provided evidence that acidolysis reactions occur between the carboxyl groups of the CPs and the amine and amide groups of PA, with formation of CP/PA graft (CP-g-PA) copolymers, although these reactions need relatively long times to go to completion. In the second part of the work, ternary LDPE/PA/CP blends have been prepared and characterized with a number of techniques. It has been shown that the addition of only 1-2 phr of CP into the LDPE/ PA blends is sufficient to enhance interfacial adhesion, to improve the minor phase droplet dispersion and to hinder coalescence. The effectiveness of the investigated CPs increases with an increase of the AA content from 6 to 11 wt.-%. Partial neutralization of the carboxyl groups of EAA with zinc also seems to improve the CP efficiency and this effect is thought to result from an acceleration of the acidolysis reactions responsible for the formation of CP-g-PA copolymers.

Thermal behaviour of poly(methacrylic acid)/poly(N-vinyl-2-pyrrolidone) complexes

Poly(methacrylic acid)/poly(N-vinyl-2-pyrrolidone) (PMAA/PVP) complexes were prepared by two different procedures: simple mixing of preformed PMAA and PVP and radical polymerization of methacrylic acid in aqueous solution in the presence of poly(N-vinyl-2-pyrrolidone) (template polymerization). Differential scanning calorimetry, thermogravimetric analysis and Fourier-transform infrared spectroscopy were performed to evaluate the properties of the two kinds of complexes and to establish if they showed chemico-physical differences that could be related to the preparation procedure. The results of our investigation indicated that the behaviour of these complexes is qualitatively similar, although it was observed that in the case of the complexes produced by blending, the process of PMAA anhydridization induced by a programmed thermal treatment of the samples, was less favoured. This suggests that by template polymerization it is possible to prepare PMAA/PVP complexes having a more ordered structure than that of similar complexes produced by simple mixing of the preformed components.

Dextran/Poly(acrylic acid) Mixtures as Miscible Blends

Bioartificial polymeric materials based on blends of dextran and poly- (acrylic acid) were prepared in form of films and characterized in order to evaluate the miscibility of the natural component with the synthetic one. Films with different composition ratios were prepared by solution casting and analyzed by dynamic mechan- ical-thermal analysis, differential scanning calorimetry, and scanning electron micros- copy. The obtained results indicate that dextran is miscible with poly(acrylic acid). The miscibility was mainly ascertained on the bases of the occurrence of a single composition-dependent glass transition temperature in each blend and also on the bases of the transparency and homogeneity of the films. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2089-2094, 1997

Bioartificial materials based on collagen: 1. Collagen cross-linking with gaseous glutaraldehyde

The effect of exposure time of thin films of soluble collagen to glutaraldehyde (GTA) vapour was studied at 37 degrees C, and was evaluated by examining the thermal and biological stability and the swelling ratio. It was found that the collagen films treated with GTA vapour for 18 h showed the highest denaturation temperature, the lowest swelling ratio, and an enhanced proteolytic stability. This study shows that soluble collagen can be effectively cross-linked with GTA vapour and that the degree of cross-linking can be controlled by varying the exposure time.

Bioartificial materials based on collagen: 2. Mixtures of soluble collagen and poly(vinylalcohol) cross-linked with gaseous glutaraldehyde

Thin films of both pure soluble collagen (CLG) and poly(vinylalcohol) (PVA) and mixtures of the two, ranging from 20-80% PVA composition were studied to test the effects of PVA content and of glutaraldehyde vapour cross-linking. Both the thermal and mechanical behavior and, in addition, proteolytic stability were clearly influenced by the ratio of CLG/PVA. The experimental results indicate that no thermodynamic compatibility occurs between the two homopolymers. However, there is evidence that strong interactions, probably due to hydrogen bond formation, occur between the biological and synthetic polymers. The interactions appear stronger in those films with a lower PVA content and which were not cross-linked. Both the thermal and biological stability are increased and there is an improvement of the mechanical properties. The mutual intermolecular influence appears to allow the attainment of a good mechanical compatibility between CLG and PVA.

Measurements of particle-size distribution during suspension polymerization

The particle-size distribution is one of the main characteristics of polymers produced by suspension polymerization which determines the final properties of the product. Since the on-line measurement of particle-size distribution during the course of polymerization is not a straightforward task, a technique has been developed to accomplish such a measurement for a predefined polymerization process. In this paper the experimental procedure is described in detail and the experimental data are shown for the batch suspension polymerization of methyl methacrylate using benzoyl peroxide as initiator, water as suspending phase and agarose as suspending agent. The particle-size distributions have been measured both for a base case and also when varying some operating conditions including stirring speed, water-to-monomer weight ratio, and suspending agent concentration. Average values and variances of the surface distribution and of the volume distribution are also computed and shown for comparison. © 1999 Society of Chemical Industry

Enzyme-based bioartificial polymeric materials : The α-amylase-poly(vinyl alcohol) system

The possibility of realizing bioartificial polymeric materials from blends of the enzyme α-amylase (EC 3.2.1.1) and poly(vinyl alcohol), in the form of film and hydrogel, was investigated. The interactions at molecular level between the enzyme and the synthetic polymer have been studied by calorimetry, X-ray diffractometry and morphological analysis. A certain degree of interaction through which the α-amylase molecules influence the crystallinity of the poly(vinyl alcohol) was observed. The thermal stability of the enzyme in the blends was enhanced, while its biological activity did not vary significantly. In addition, hydrogels prepared by a freeze–thawing method can be considered suitable for manufacturing delivery systems capable of releasing enzymes which maintain their full biological activity. ©1997 SCI

Dehydro-thermally cross-linked collagen-poly(vinyl alcohol) blends: mechanical, biological and surface properties

The mechanical, biological and surface properties of films, based on blends of soluble collagen and poly(vinyl alcohol), were investigated. Films prepared by casting were cross-linked by a dehydro-thermal treatment. The mechanical behaviour of the materials was studied by dynamic-mechanical thermal analysis. The biological properties of the films were evaluated by performing cytotoxicity and cytocompatibility in vitro tests. Surface characterization was carried out by X-ray electron spectroscopy. The results obtained indicate that these materials behave as two-phase systems and a remarkable enrichment of collagen in the surface with respect to the bulk was observed. Cytocompatibility tests show that the blending of the two polymers creates a better substrate for cell growth in comparison with pure components.