14. The maximum value of the function $f ( x ) = \sin ^ { 2 } x + \sqrt { 3 } \cos x - \frac { 3 } { 4 } \left( x \in \left[ 0 , \frac { \pi } { 2 } \right] \right)$ is ______
14. The maximum value of the function $f ( x ) = \sin ^ { 2 } x + \sqrt { 3 } \cos x - \frac { 3 } { 4 } \left( x \in \left[ 0 , \frac { \pi } { 2 } \right] \right)$ is ______\\