About Polymers In General

About polymers in general

A polymer is a macromolecule composed of many repeated units, named monomers. The word is formed by “poly-” which means in greek “many” and “-mer”, which means “parts”. Such a linked backbone as a polymer has multiple applications in everyday life. The familiar synthetic plastic as polystyrene or polyvinyl chloride (abbreviated PVC) to natural structures such as DNA (deoxyribonucleic acid) and proteins are all polymers. In fact, these latter are the longest polymers from the living world.

The properties of every polymer depend on the identity of its constituent monomers and their arrangement within the polymer. This arrangement along the backbone of the polymer is called configuration. The chain can deviate from a simple linear chain to a different configuration, so each polymer has its own architecture and shape, which comes with different properties. When we refer to properties, we mean: viscosity, solubility in various solvents, glass transition temperature (Tg), resistance to flow, strength, toughness etc. If the chain length is increased, the chain interactions are increased and that because of the Van der Waals forces between molecules. Imagine molecules as being surrounded by a cloud of negative electrons which repel one another as two polymer chains approach. The effect is lowering the electron density on one side of a polymer chain, thus creating a slight positive dipole on this side. This charge is enough to attract the second polymer chain, thus increasing the chain’s length. Increasing chain length means decreasing chain mobility, increasing strength and toughness. The ionic bonding or hydrogen bonding can appear even between different side groups of the polymer. These stronger forces give higher tensile strength and higher crystalline melting points. [a]

-despre efectele luminii asupra polimerilor (3 pagini)

Din [d] http://www.icmpp.ro/mcps/files/Raport%20stiintific%202013.pdf

[c] L. M. Constantinescu, “Structura polimerilor”, Editura Universității din București (1989)

Behavior of the polymers due to the action of the environment

There are three important ways in which polymers can be modified: oxidation, cross-linking and end-capping. [b] We will talk first about the processes which appear when the polymer is irradiated.

UV exposure

[c] Most of polymers are affected by radiation. Exposed polymer systems bear photodegradation processes, depending on the wavelength and the intensity of the radiation, the time of exposure and the chemical structure, respectively. These processes imply aspect modifications (colour, brightness) and structural modifications (ruptures of the macromolecules ) which affect the mechanical and physical properties. [c]

[d] The solar light and especially the UV portion of the light spectrum is responsible for the initiation of the photochemical degradation. The photodegradation of the polymers is an effect of energy dissipation of the excited molecules. The process can determine the rupture of the excited macromolecule and the appearance of new macromolecular fragments, which modifies the polydispersion of the system and decreases the molecular mass of the polymer. On the other hand, between the chains of the polymers may form chemical bonds which give result of reticular structures (cross-linking process), developing a larger molecular mass and decreasing solubility. The process of photodegradation continues through secondary processes (e.g. oxidation) where oxigen and other substances formed in the first process participate with the polymers chain to new bonds formation. [d] The polymers present in the chemical reactions a series of particularities which result from the place the functional groups occupy in the macromolecule, depending on the partial or full participation of the macromolecule to the reaction and on the type of reactions which determine the functional groups. [c, e]

[e] L.M. Constantinescu, C.I. Berlic, “Structura polimerilor. Metode de studiu”, Editura

Universității din București (2003)

Iradiation

Ionizing radiation may affect the properties of the polymers in a negative way (destruction through irradiation ) or in a positive way, which leads to an improvement of some properties.

(din

[f] A. Chapiro, Radiation Chemistry of Polymeric Systems, Interscience Publishers, John Wiley ans Sons, New York – London, 1962;

[g] A. Charlesby, “Atomic Radiation and Polymers”, Pergamon, London (1960) )

In the case of cross-linking, the molecular mass of the polymers increases proportional with the dose and a tridimensional lattice is formed. On the other hand, when the polymer degradates, the molecular mass decreases with the dose. Some polymers contain secondary or tertiary carbon atoms and suffer reticular processes, while the polymers with quaternary carbon atoms suffer a degradation process. [h, i]

(din

[h] W. Schnabel, “Polymer Degradation; Principles and Applications”, Macmillan, New

York, 1981

[i] S. Horun, O. Sebe, “Degradarea si stabilizarea polimerilor”, Editura Tehnica, Bucuresti,

)

Degradation involves losing of some properties and cross-linking leads to improved properties.

Autoinduced cross-linking

Autoinduced cross-linking (radiochemical) is produced by irradiating the polymer with X or gamma rays. In this process free radicals with short time-life are formed which bring together the macromolecules’ chains through transversal bonds, with the forming of hydrogen as a secondary product. The transversal bonds’ formation takes place in the amorphous zone of the polymer. The X and gamma rays’ energy is great enough to break the O-O, C-C, C-O, C-H etc. bonds. The cross-linking process modifies the mechanical and thermal properties. [j, k]

Din

!!! [j] “Raman Spectroscopy of Gamma Irradiated Polypropylene – Carbon Nanofiber Composites,”

[k] H. C. Biggin, “Iradiation Effects on Polymers”, Ed. D. W. Clegg, A. A. Collyer, Elsevier Applied Science, London and New York, 1991, Ch. 1

Autoinduced degradation

!!! din efectele radiatiilor asupra polimerilor

The degradation process consists in losing some of the physical, mechanical and chemical properties, changing the colour and sometimes in cracking the polymer material. The polymers behave differently to the action of destructive factors. This difference appears because of the dependence of the photodegradation process on chemical composition, microstructure, energy of the chemical bonds and on molecular mass of the polymer. Irradiation brings important modifications when it takes place in the presence of oxygen. [l, m]

Din

[L] D. W. Clegg, A. A. Collyer (ed.), “Irradiation Effects on Polymers”, Elsevier Applied

Science, London and New York, 1991

[m] T. Setnescu, R. Setnescu, S. Jipa, I. Mihalcea, Polym. Degrad. Stab., 52, p. 19, 1996

Oxidation is a particular and complex process, because of the various functional groups that can interact with the oxygen from the medium. Oxidation reactions determine slow modifications of the initial structure of the polymer and alteration of its properties. Have been found that even after the irradiation is stopped, the oxidation continues in an accelerated way, because the formed free radicals respond to the oxygen from the medium. [L] The reaction of oxygen with free radicals increases the rate of rupture reactions of the macromolecular chain. However, the presence of oxygen contributes to forming –C-O-O-H- transversal bonds between the macromolecular chains, but these bonds are not thermally stable. For this reason, the polymers resist less to irradiation when it takes place in the presence of oxygen.

Humidity

Under the action of humidity, the polymer degradation is fast. Due to this process, free radicals can result, which amplify the degradation reactions and affect the thermal stability and the structure of the polymers.

Din [d] http://www.icmpp.ro/mcps/files/Raport%20stiintific%202013.pdf

Poly-ethylene glycol

Din [b] https://en.wikipedia.org/wiki/Polyethylene_glycol

Polyethylene-glycol is a polymer compound with many applications in: industrial manufacturing, chemistry, biology, medicine etc. The structure of PEG is H-[O-CH2-CH2]n-OH (Fig.1, Fig.2).

Fig.1.2.1. The chemical structure of polyethylene glycol

Fig.1.2.2. Polyethylene glycol – linear fragment

PEG is preferred in the biomedical field, presented in two forms: polyethylene oxide (PEO) and polyoxyethylene (POE) which are chemically synonymous, but their use depends on their molecular weight. For example, polymers with molecular mass below 20 000 g/mol refer to PEG. PEG is presented in liquid or solid form with a low-melting point. An image of PEG is presented in the experimental chapter. Every PEG is characterized by a molecular weight presented in the name – for example PEG-400 for a molecular weight (MW) of 400 g/mol. [b]

PEG is in the first category, thus reticular structures are formed through irradiation. [h]

PEG can be attached on different molecules, for example a protein or a nanoparticle. The process is named PEGylation (end-capping).

PEG is used commercially in numerous applications, including as surfactant, in foods, in cosmetics, in pharmaceutics, in biomedicine, as dispersing agents, as solvents, in ointments, in suppository bases, as tablet excipients, and as laxatives. [b]

-despre nanoparticule si GNP in special (2-3 pagini)

About nanoparticles

It is known that light is an electromagnetic radiation; this means that it is described by the electric and magnetic field, moving with a certain speed c. The Lorentz model treats charged particles from a material as harmonic oscillators. When light interacts with a small object, the electric field drives harmonic motions of electrons. If the small object is a metal nanoparticle, there takes place an absorption at resonance frequency of the surface plasmons.

Din [n] http://www.sigmaaldrich.com/materials-science/nanomaterials/gold-nanoparticles.html

[o] http://www.cytodiagnostics.com/store/pc/Gold-Nanoparticle-Properties-d2.htm

Nanoparticles are made from noble metal: Au, Ag, Pt, Pd etc. When a radiation of a certain wavelength interacts with the surface of the nanoparticle, the free electrons start to move in the opposite direction of the electric field. (Fig.2.) But this means that the positive charge is driven in the same direction of the electric field. This leads to another electric field between these amounts of charges, which tries to make the electrons to come back. Thus, the electrons oscillate. If the wavelength of the collective oscillation is the same with the wavelength of the

Fig.2.1. Basics of localized surface plasmon resonance (LSPR) of gold nanoparticles due to collective oscillation of surface electrons with incident light at a specific wavelength.

radiation given, the surface plasmon resonance phenomenon appears. [n] This phenomenon is characterized by strong extinction of light (absorption and scattering). The wavelength of light where the phenomenon occurs is strongly dependant on the gold nanoparticle size, shape, surface and agglomeration state. [o]