Maurice Brogly

Effect of vinylacetate content on crystallinity and second-order transitions in ethylene—vinylacetate copolymers

Thermal characteristics of ethylene—vinylacetate (EVA) copolymers having vinylacetate contents ranging from 5 to 40 w/w % are studied by differential scanning calorimetry. It is first shown that EVA copolymers having a vinylacetate content lower than 30 w/w % obey the Flory and Burfield theories of copolymer crystallisation. The minimum sequence length of CH2 ethylenic entities required to participate in a crystalline lamella is also deduced. One can conclude that EVA copolymers represent cases of “total exclusion” of the noncrystallizable comonomer. Moreover, it is observed that when the vinylacetate content is increased, the relative quantity of polyethylene amorphous phase increases and the degree of crystallinity decreases; whereas the β transition temperature of a characteristic-oriented amorphous phase is kept constant. A phase model of ethylene-vinylacetate copolymers, based on an enrichment process of the interlamellar amorphous phase by polyethylene segments originating from the crystalline phase, at increasing vinylacetate content, is proposed.

Crystallinity measurements of polyamides adsorbed as thin films

The crystallinity degree of thin polyamide films (PA66, 610 and 612) spin coated on four types of substrates are estimated. Two original methods based on the analysis of infrared spectra are proposed. The high sensitivity of polarization modulation reflection-absorption spectroscopy for the study of thin polymer films leads to quantitative results. These two methods offer the possibility to estimate the degree of crystallinity by using infrared spectroscopy, independently of any other technique, if the crystallinity degree in the isotropic bulk state is known. Polyamides are spin coated, on the one hand, on inert and highly reflecting gold substrates, and on the other hand, on the same gold substrates grafted with different chemical functionality. Our calculations show a lower crystallinity degree for inert substrate and higher values for functionalized systems with respect to the bulk. These results suggest that the surface chemical grafts not only play the role of nucleation seeds, but also affect the crystalline morphology as seen by atomic force microscopy.

Quantitative calculation of the orientation angles of adsorbed polyamides nanofilms

Abstract Polyamides (PA) PA66, 610 and 612 were adsorbed as thin films on functionalized substrates. The conformational changes after adsorption are studied. New conformations due to the rotation of diamine/diacid planes were evidenced by infrared reflection absorption spectroscopy (IRRAS) and polarization modulation IRRAS (PM-IRRAS). The study of some infrared vibrators, which have a well defined transition moment direction with respect to the principal chain axis, allows us to access a qualitative information on the molecular orientation of PA chains. In this paper, we will discuss the possibility to obtain quantitative data on polyamide conformations on the basis of PM-IRRAS experiments. Values of orientation angles of diamine/diacid planes versus surface plane were determined. Similar angles are found in the four cases of gold, OH, NH 2 and COOH functionalized substrates, and for the three PA. Twist angle of about 20° is found between diamine and diacid planes. The chain orientation angles and conformations obtained do not depend on the substrate functionality and thus suggest that they are only the consequence of an adsorption effect.

Chain flattening of spin-cast PMMMA on aluminum mirrors: Influence of polymer tacticity

Abstract Polymer tacticity has already been shown to be an influential parameter governing the behavior of polymer solutions and polymer blends, but not for polymer adsorption. However, using PMMA having a racemic fraction (ƒr) ranging from 0.1 to 0.9, spin-cast on aluminum mirror surfaces, makes it possible to work out configuration differences in the adsorbed layer. Reflectance infra-red spectroscopy has been used to evaluate the fraction of surface bonded carbonyl groups (ƒabCO) and local conformations of both main and side chain. The ƒabCO value is 0.1 for highly syndiotactic PMMA and reaches 0.7 for mainly isotactic PMMA. This latter very high value of ƒabCO can only be explained by chain flattening resulting from a distortion from its Isotropic state. This distortion may be due to both strong interactions that may exist between PMMA and the aluminium substrate, and also to the spin-cast process that may lead to chain straightening and flattening. The assumption of conformational energy changes of both backbone and side chain is confirmed by changes in the FT-IR Av 2 (1245 cm −1 ) Av 1 (1265–1275 cm −1 ) ratio. Ellipsometric spectroscopy has provided information on the global conformation of adsorbed chains by precise thickness measurements. Thus, thickness (h) increases for diluted solutions linearly from 10 to 70 A as the average syndiotactic sequence length (ns) increases. However, IR and ellipsometric results tend to prove that isotactic PMMA may adsorb on the aluminum surface in a more flattened way with longer chains than atactic and syndiotactic chains. This can be explained by the ability of chain segmental rearrangements (meso tg state) that may allow isotactic PMMA to favor the establishment of interfacial acid-base bonds while the syndiotactic PMMA local stiffness of the tt state does not. The influence of the chain entropy in solution is also discussed as an important parameter.

Evidence of Acid-Base Interfacial Adducts in Various Polymer/Metal Systems by IRAS: Improvement of Adhesion

Abstract The physical interactions of polymers with inorganic substrates are determined by two major contributions: Van der Waals forces and acid-base interactions, taken in the most general “Lewis” electron acceptor-donor sense. The present work shows that the work of adhesion can be very appreciably increased by the creation of interfacial acid-base interactions. Practically, polymers such as poly(ethylene-co-vinyl acetate) (EVA), terpene-phenol resins (TPR), and their blends, were solution cast on basic and acidic substrates. The nature of the interfacial bonds and the enthalpy of adduct formation through electron exchange are evidenced by Fourier transform infrared reflection-absorption spectroscopy (IRAS). Moreover, it is shown that, on the one hand, modification of the electron donor ability of the polymer functionalities reveals the amphoteric character of the substrate and, on the other hand, modification of the electron donor ability of the substrate changes the nature of the species involved in ...

Influence of tacticity on the conformation and development of acid-base adducts of PMMA homopolymers adsorbed on aluminium mirrors

Abstract The influence of tacticity on (1) the conformation of adsorption at a polymer/substrate interface and (2) the magnitude and density of acid-base interactions developed at the interface, has been studied. Films of poly(methyl methacrylate) (PMMA) homopolymers of various tacticities were spin-coated on aluminium mirror substrates. Ellipsometric measurements show that the PMMA film thickness increases linearly with the syndiotactic sequence length (as determined by 13C nuclear magnetic resonance spectroscopy.) These results tend to prove that the backbone conformation, the spatial extension and the segmental rigidity are affected when the density of racemic duds increases. Fourier transform infra-red reflection-absorption spectroscopy (IRAS) reveals orbital overlapping between the sp bonding orbital of the PMMA carbonyl oxygen atom and the σ∗ antibonding hydroxyl orbital and allows us to calculate an enthalpy of interaction of −10·5kJmol−1. Moreover, it is shown that for low amount of adsorbed polymer (10−2M), the density of acid-base interfacial adducts decreases as the average syndiotactic sequence length of the PMMA increases. This tends to confirm that the conformations of adsorbed isotactic PMMA chains are more extended than those for syndiotactic chains. IRAS analysis shows that the development of interfacial acid-base adducts induces backbone trans-gauche conformations changes in trans-trans conformations, leading then to an increase in the conformational energy of the main chain. This effect is magnified as the isotacticity of the adsorbed PMMA homopolymer increases.

A new theoretical approach for the determination of molecular orientation persistence length of adsorbed nanofilms by FTIR reflectance spectroscopy

FTIR-Reflectance experiments have proved to be a powerful tool for the determination of molecular orientation in thin films adsorbed onto highly reflecting metals. We propose an original method for determination of the persistence length of molecular orientation in the film. This approach is based on the fact that molecular orientation persists only over a given distance from the geometrical interface. This distance is called the persistence length of molecular orientation. We then suppose that the nanofilm adsorbed is stratified and consists of an oriented layer (in the near-interface region) plus an isotropic one. Correlation between infrared reflection absorption band intensities and simulated band intensities allows experimentators to determine accurate molecular orientation and persistence length of orientation of a considered functional group. This is accomplished by using various IR reflection angles and p-polarization state of the incident IR wave. Film thickness and complex refractive index spectra, n(ν) - i.k(v), are only needed to deduce calculated specular reflectance intensities.

Mechanisms of Interfacial Degradation of Epoxy Adhesive/Galvanized Steel Assemblies: Relevance to Durability

This study reports on the durability of galvanized steel joints bonded with toughened epoxy adhesive formulations under the influence of temperature and moisture aging. The influence of hydrothermal aging on the substrate and adhesive properties was first determined and then correlated to the durability of bonded joints. The mechanical properties of the adhesively bonded joints were assessed by a shear stress-strain analysis on single lap joints. Experimental results have shown a loss of adherence during aging. Scanning electron microscopy, fourier transform infra-red spectroscopy, and differential scanning calorimetry were conducted to analyze precisely the locus of failure and the failure faces after separation of the interface. Consequently, physical and chemical modifications of the adhesive and surface corrosion of the substrate were observed. A three-step mechanism has been proposed to explain the loss of adhesion during aging. The first step was attributed to adhesive and interface hydrolysis of the lap-shear samples Highly stressed zones, the second step to hydrolysis of the centre of the bonded joint and initiation of corrosion. The last one is due to an important growth of corrosion products which favours rupture in a weak boundary layer at the metal/polymer interphase. The aging model proposed bears out that the higher the corrosion resistance of the metal surface the lower the loss of adhesion under a wet environment.

Force Curve Measurements with the AFM: Application to the In Situ Determination of Grafted Silicon-Wafer Surface Energies

The atomic force microscope (AFM) can be used to perform surface force measurements in the quasi-static mode (cantilever is not oscillating) to investigate nanoscale surface properties. Nevertheless, there is still a lack of literature proposing a complete systematic and rigorous experimental procedure that enables one to obtain reproducible and significant quantitative data. This article focuses on the fundamental experimental difficulties arising when making force curve measurements with the AFM in air. On the basis of this AFM calibration procedure, quantitative assessment values were used to determine, in situ, SAM (or Self Assembled Monolayer)-tip thermodynamic work of adhesion at a local scale, which have been found to be in good agreement with quoted values. Finally, determination of surface energies of functionalised silicon wafers (as received, CH3, OH functionalised silicon wafers) with the AFM (at a local scale) is also proposed and compared with the values obtained by wettability (at a macroscopic scale). In particular, the effect of the capillary forces is discussed.

Adsorption of Alkanethiols on Gold Surfaces: PM-IRRAS Study of the Influence of Terminal Functionality on Alkyl Chain Orientation

The present paper focuses on a simplified method to study the orientation and the anisotropy of two different alkanethiols self-assembled monolayers on gold surfaces. The alkanethiols of interest vary only by their terminal functionalities (COOH and COOCH3), thus allowing one to highlight the influence of these ending chemical groups on the final orientation of the adsorbed molecules. 11-Mercaptoundecanoic acid [HS-(CH2)10-COOH] and the methyl-11-mercaptoundecanoate [HS-(CH2)10-COOCH3] were grafted under adequate conditions to obtain a high grafting density on gold substrates. These latter, before and after the alkanethiol adsorption, were analyzed mainly by the polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) technique to access orientation angles, and by atomic force microscopy (AFM) to check the homogeneity of the grafted layer. By applying an original PM-IRRAS simplified method, the results showed an orientation closer to the normal of the surface plane in the case of the acid thiol compared with the ester one, which is probably because of the lateral hydrogen bonds established between the adjacent COOH functions. This method offers a direct and simple way to quantify the orientation angles in the alkanethiol nanofilms.