# Bitumen and Bitumen Modification: A Review on Latest Advances
## 2. Bitumen Polymers  
Polymers are macromolecules synthesized through chemical reaction between smaller molecules (monomers) to form long chains. The physical properties of the resulting polymer are determined by the chemical structure of the monomers and by their sequence inside the polymer. A combination of two different monomers that can be in a random or block arrangement gives a so-called copolymer. Polymers include a broad range of modifiers with elastomers and plastomers being the most commonly-used types. Polymer-modified bitumens (PmBs) are produced by the mechanical mixing or chemical reactions of a bitumen and one or more polymer in a percentage usually ranging from 3% to 10%, relatively to the weight of bitumen. In the first case, the mixtures are said to be simple, because no chemical reactions occur between the two partners in the system. In this case, the polymer is considered as a filler which gives specific properties to the mixture. In the second case, the mixtures are said to be complex, because chemical reactions or some other interaction occurs between the two partners in the system [49].  
Modified bitumens are characterized as a two-phase system: bituminous, prevalently as asphaltenic matrix, and polymeric matrix, which has been investigated from two different points of view.: (1) the complex interaction mechanism between bitumen and additive and (2) the influence of different type of bitumen modifiers aiming to study the rheological performance characteristics, temperature sensitivity, morphology, thermal behavior storage stability, and aging of the resulting PmBs.  
From a bitumen/polymer interaction mechanism point of view, according to [PERSON] et al. [50], polymer modification results in a thermodynamically unstable but kinetically stable system in which the polymer is partially swollen by the light bitumen components (maltenes) and can swell up to nine times its initial volume [51]. Competing for bitumen's light fractions, polymers tend to induce the micelles aggregation of the asphaltenes or to increase their degree of association, according to the nature of the original bitumen. At high temperatures, a relatively low viscosity of the melted micro-heterogeneous polymer-modified bitumen allows the substances with similar structure and polarity to form their domains: the swollen polymers and the asphaltenes. However, the thermodynamic instability of this system induces a phase separation (or sedimentation) under the influence of the gravitational field. Therefore, associated asphaltene micelles can settle to the bottom of the blend during static hot storage. According to this mechanism, the degree of phase separation of polymer modified binders can be influenced by storage conditions such as temperature and time. As shown by [PERSON] et al., the phase separation will mainly be governed by the nature of the base bitumen and the characteristics and content of the polymer [8]. To date, different types of additives and polymers have been used for bitumen modification [52]. Table 1 summarizes the most common types of additives used as bitumen modifiers.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  
In Table 2 are summarized the most common used modifiers found in the literature, which are discussed in this paper.  
Each of these groups associate with different pros and cons as a bitumen additive. In addition to the large group of polymers, other bitumen modifiers, such as polyphosphoric acid (PPA), sulfur, maleic anhydride, and different kinds of clays, have been introduced and experienced, in this respect, some success.